I've done some digging around on the internet and I think I know the answers, but I just want to double check since I haven't found the exact answers.

I've got an '84 MK 3 Cabrio project car with a 1.6 MFI engine in it that is leaking and sweating oil out of pretty much every gasket/seal. I've also got access to an '87 MK4 parts car with a 1.6 carb engine in it which in general looks in much better condition.
Could I just put all the MFI related parts on the carb engine and put that one in the MK3? The original plan was to rebuild the original engine but then I could still do that afterwards while still have a running car.
I know there's a difference between the older and newer heads (the MK3 engine has an 81SM6090 head and the MK4 one an 88SM6090), but since neither are EFI as far as I know it should be ok.

Apart from the MFI related parts, apparently the MK3 and MK4 engine mounts and gearbox housings aren't identical either. I assume I can just bolt the MK3 mounts and gearbox to the MK4 engine to fix that?

I've considered swapping to a Zetec or something else but decided I want to keep it more or less original.

 

Mk3/4 mounts are interchangeable on the engine. Gearbox the casing is physically different, so just keep the mk3 box fitted.

 

That's mostly a direct swap. Depending on how old your MK3 engine is you'd want to keep the MK4 (when having your MK3 engine rebuilt) head as these are suitable for unleaded fuel.

Camshaft profiles are different, so keep your MFI camshafts and followers. Do the same with the rest of the MK3/MFI parts.