fitzy's right , adam is the man to ask
but basically you need to change EVERYTHING
in chassis terms you will need a mk2gti subframe altho it leaves the engine very close to the bonnet, you can us mk3 one too for 30mm(?) wider track
i wouild recommend 256mm brakes at least as well,
you can use either the mk3 box which is cable change, or mk2 16v box But you will want one with stiffened diff pins (normally done wen they are rebuilt) or you will break it.
or tdi (mk3 )box cable not rod if you crave 150mph +
i your position it might be easier to put and abf in on twin45's?? or equivalent
R1 carbs fit straight on ??
check out clubgti forums for the in depth tech pretty dam sure its all on there
my thoughts 'outside the box'