I remember when 4AGE's first came out, they were touted as a non rebuildable motor. Laughable now, but back in 1985, there were a few worried car club people around!
Anyway, 16V are very easy. Only hard bit is the engine management if you exceed the limits of the factory ECU, which you don't need to do.
20V in a FWD configuration isn't hard to fit, but a bit of mucking about adapting looms to an earlier generation chassis. Don't underestimate how complex this is to get right.