2g CRX, 4g Civic K Series Engine Swap
Starting things off I'm going to do a breif overview.  A K series engine swap is no harder than any other.  There are a few more things to address, but it really isn't that hard.  This swap will require welding of the  passenger side mount, and modification of the rear crossmember.  The new shift linkage will also be obviously engine mounts will have to be changed.  The only manufacture making mounts for this swap is HASport

First off pull the engine, and remove all the wiring from the bay, and pull it into the cabin.  A K swap is the perfect reason to do a wire tuck since none of the engine wiring is reused.  For the wiring, you have 3 choices, custom,
HASport, or Rywire.  I chose Rywire for he custom builds them for his customers, and it is plug and play.  He made mine for me to turn on my fan with the Hondata, and made it to work with the wire tuck.  Newer Hondas have immobilizers, and Hondata K pro does away with that, plus it is programable.  The other option is a non programable JDM ECU. 

Now that the engine bay is empty it is time to remove the passener side mount from the engine bay.  This can be achieved by using a spot weld cutter or other methods.  Next you bolt the mount tower to the frame rail with the 12mm battery tray bolts.  Figure out where you need to grind, mark it, remove the tower.  Grind it, reinstall the tower, and weld in the tower. 
HASport says just to weld in the holes, but I seam welded the whole tower in.   That is what everyone I know has done aswell.

Welded in the mount should look like this.  Not also there are other areas I welded holes shut for a smooth shaved bay.  It is allot of extra work, and not worth it in my opinion, but I went the extra mile. 
Next we shift our focus the the rear crossmember the area in the squares needs to be dented.  The blue area needs dented allot like 1-1.5"  I also cut the upper lip, and seam welded it shut.  I did the same to the other side too for the header clearence, but it really wasn't necesary with my SSR header I used.  Speaking of headers the SSR, and the DC (HASport) header are the only units I know of to work on the CRX chassis, although SMSP is working on one that might work.  SSR offers a 4-1, and a 4-2-1.  I have the
4-2-1 it is what most use for more midrange power.
At this juncture your engine bay should look like this.  I bolted in the rear mount, the pass side tower is welded in, and everything is prepped, and ready to paint, well mabey not in this pic.  At this point I would reccomend installing the engine for a test fit, and then working out any clearence issues.  Thats what I did, and I had to dent my crossmember more.  Plus it gave me a chance to work on my EP3 Shifter assembly, to make sure it would work well.  You will also notice the Z10 traction bar setup.  A stock crossmember will not work with a K swap.  Z10, Full-Race, and Explicit Speed crossmembers are known to work.  Jim fab bars, that are no longer made, don't work.  I also had to cut my headlight bucket to clear the alternator for the 3 pulley setup. 
Speaking of shift linkage that is something that is to be addressed as well.  Lots of people use the DC5 Integra unit, and they make kits to mount them to the floor, or some modify the floor, and weld it to accept the shifter.  I on the other hand, had preference for the dash mounted shifter in the EP3(02-05 Civic Si).  So I purchased a Buddy Club short shifer assembly, and EP3 shift cables.  You cannot use DC5 cables, with the EP3 or vice versa.  Most people will not go this route, and will use the DC5 stuff.  I will add that info later but for now this is how I did mine.  I fabricated mounts, and welded them to the radio mounting area.  Oh yeah my car has no heat.   I'm pretty sure if you were to retain heat this would be very dificult if not impossible.  I actually ran my shift cables through the heater core holes .
You might have noticed all of my brake lines were removed.  I relocated my brakelines under the dash, and mounted the prop valve there aswell.  You don't have to do this but I did for a cleaner install.  I did this after I painted my bay, and before I reinstalled the engine for the final time.  The other option is to bend the lines, and have it hang back.  Once again I didn't do that but I will add info on how to do it on another date.  So now we install the engine, and start hooking stuff up, thats where we are at now.  The Rywire harness is simple.  You run the engine harness into the cabin, and plug it in.  You have to run the o2 sensor, and the fan switch wires out, and connect them.  Next you will have to adress the fuel lines, radiator, intake, install your header and axles.
The fuel system is simple, but a little costly.  I ran a fitting  On the OEM fuel feed, and adapted it to 6AN, then I ran AN lines to an Areomotive filter, and then to the regulator.  From the regulator you run to the rail, and plug the other end.  The return line, then comes from the bottom of the regulator.  I used Aeromotive for this aswell, but AEM is another good choice.  Whatever you do don't buy a cheap filter I got one from Jegs and it was a peice of crap and clogged almost immediately.  Now for the rad in my above pic you will notice the ugly peice I used.  I since then converted to a 91 VW GTI non-AC it fits like a champ.  I also had to use a inline fillneck to fill  for the rad is made for a remote fill.  The rad will have to be custom mounted.  I got all the fuel stuff from Jegs.
Here is the GTi Rad, and the custom hoses, and fill neck.  The GTi rad is mounted upside down, and the sensor hole will need plugged for it is not needed.  Also Note the custom Air Intake.   I built it from a 3" U bend cut, and welded, and a 3" K&N filter tops it off.  Most people just make them sit in the engine bay sucking in all the hot air.  I wanted to do away with that so I made it go where the washer tank was.  I have since scrapped this, and I'm building a custom airbox, with a ram air velocity stack feeding it.  Also note the catch can installed running to the PCV valve, and the custom oil cap to clear the hood courtesy, of John Ward.  All the little parts were from Jegs, once again. 
Next install the axles, DSS are what I used,, fill up the fluids, and connect the brake booster vaccumm line.  Double check all of your work.  Go have some fun.  This motor is tall and will hit an OEM hood.  I have a JDM hood with trimmed webbing and it fits just fine.  Fiber Images offers hoods for the USDM, and JDM frontends, and they clear aswell.  This is only true with a k20, k24s will require, a custom hood.  Some other items you should install would be some upgraded brakes, and a car alarm.  Oh and as far as I know the only strut bar that clears the K series is a Cusco Type-ST.  For your speedometer to work you will either need to use a electronic VSS cluster or, a speed sensor from a cable speedo tranny.   Your exhaust will have to be modified to fit the K header.  It is in a totally new location.  I hope you enjoy this, and found it insightfull.  If you need anything I can be reached by email.
Price List
K20A Complete Swap                          
Six Sigma Racing Header                          Hondata K Pro                                       
HASport motor mounts                          
Rywire Harness                                       
Drive Shaft Shop Axles Stg 1                 
Buddy Club Shifter EP3                           
Aeromotive Fuel Regulator                     
Fuel Lines, and fittings                            
STR Pulley Kit                                           
91 GTi Non A/C Radiator                          
Aeromotive Fuel Filter                             
Radiator Fill neck, and Hoses                   Air Intake filter and 3" tube                      

Ok now it is time for a little preaching.  I obviously went the most expencive route on the engine.  The K20A is about $1200 more than the A2 from most importers.  For the extra $1200 you get a LSD, lower FD, 4.7, over a 4.3, a half a compression point, and some bigger cams.  Worth the extra money in my book.  Also it should be told I did not spend $10,035 on my swap.  I should have, but I didn't I got the first three items, and the axles, and pulley for $6500 picked up used off of a friend on K20A.org.  There are deals to be had if you look around.  Just don't go cheap where it matters.  I also got a deal on the Hasport mounts too from a H-T member.  The Hasport header is cheaper, but makes less power, my fuel system is expensive, but bullet proof.  My axles, could be better, and soon I will upgrade to stage 2.9's. 

Also Checkout Ryan's write up here!