Properly?
Lots of topics on this subject on the forum, if you search. ecu, tach, cas, etc.

I found a bunch on putting 90 engine into 91 but wasn't sure if you can just do the same with the 91 into the 90.
Thanks, Sleeper1ga

i did the same and had to use a 90 TB,PTU,coilpack,cas i may be forgetting other things but this will get you on the right track

My opinion, the last thing you want is the 90 TB, ISC, and TPS so you should modify the engine harness to support the 91+ TB parts.

The 90 instrument cluster doesn't function well with 91+ PTM, and ECU so you have to decide based on which ECU you have and if you still have any of the 90 ignition left how much you want to change. The 90 coils just have the tach module attached otherwise they are the same physical coil and used later. The 90 PTM doesn't have a tach output since it's on the coil pack so it has 1 less pin than the 91+ PTM.

If you still have the 90 engine, you want to keep the 90 oil cooler, so clean it out and swap the filter housing to the new engine.

But the easiest is to use the '90 parts (ecu, tb, isc and tps) so you will have to decide what way to go. I don't have a problem with my '90 parts and your car is already set up for the '90 stuff. Tough choice. Good luck!

If i were you i would switch out all the 90 parts for the 91 plus parts as parts are more accessible which is a plus on a 25 year old car (just relised this is the first year we can officially plate a 90 as a classic!) If you decide to swap the parts out please be sure to do all of it right with solder shrink wrap and be sure its done right ir your gonna have a mess

If you have both the 90 and the 91 harness its not to hard.. Up date as much as you can don't get in a hurry and do it right.. Painless as well as others sell new weather connecter's. When in doubt replace with new.

I have an aem series 1 programmable ecu. Is there anywhere I can buy the whole harness(91) new or would I need to find a parts car? Also would my dash work correctly with the 91 parts

Could possibly wait until you have the ability to post in the WTB section.