I've gone the other way which is 21 engine in my MC18, the main difference is the gear position sensor on the 21 engine which will have a block connector, I expect you can locate the neutral wire in the 21 loom and wire it back to the 18 neutral switch, or just leave it disconnected if youre not fussed about neutral light. Other than that I guess it comes down to PGM choice, obviously if your talking about MK1 PGM like I have then your also into PV control units, which can be good or bad depending on how you look at it. Bad in that its 2 other electrical things to go wrong, but good in that the PV controller wont fry your PGM. 21 ignition timing is also more advanced on 21 so you may want to think about what pick ups/flywheel you use.