Well, swapping a Primera engine into a 91 classic is just a bolt in affair. It's basically take the old out and put the new in, so it's easy.
<Edit>Err...let me rephrase that...it is easy relative to the other possible swaps.
Swapping a GTIR or BB DET in place of a 1.6 liter is a whole nother ball game. One of the motor mounts are different. You have to get a new crossmember and new axles because the 1.6 ones won't work. You'll probably want to upgrade the brakes, but that's up to you. So mechanically, it still is not hard, though it's more involved than swapping into a 2.0l. The wiring, however, from what I've been told and what I've seen is quite a mess. I hope you will either make your own wiring harness or you really enjoy splicing because no factory wiring harness will work, I think. You have to combine the 1.6 and 2.0 wiring harnesses together.
I bet someone from SR20Development or Jay Whiteley could be more specific, but I'm pretty sure that this is correct.
It has been done though, so hat should let you know that it's do-able.
[This message has been edited by tommy (edited 10-01-2001).]