You'll probably want quite a bit from the Mk3, engine, loom, clocks, ecu and possibly gearbox.
There's not going to be a massive performance gain from that swap, so are you just doing it because it was the cheapest available replacement or because you wanted a newer lower mileage engine?
If you want performance gains, you might be better looking at either the 1.8 16v engine from the Mk2 GTI (which will be less hassle to fit as it's K-jet injection) or the 2.0 16v ABF engine from the Mk3 GTI.
Anyway, if you use your Mk1's gearbox you won't have to swap axels and hubs over. If you pulled out the Mk3's gearbox with axels and hubs intact, it would then leave you with 5 stud hubs, meaning you couldn't fit the standard Mk1 wheels back on.
I can't remember, but if the Mk3 GTI g/box is cable change then it would be easier to use the Mk1 or a Mk2 gearbox. Also to consider is the clutch, if the Mk3 clutch is hydralic, then it would be easier to stick with the Mk1 clutch otherwise you'd have to convert the car to hydralic clutch.
btw, I'm no mechanical genius, I'm just re-gurgitating what I learnt on this site
Seriously though, try a couple of searches, theres loads of info on here.
