all MK1/2 1.8 bottom ends are the same, GTI, Carb etc etc. I think the 1.8 spi bottom end out the MK3 will work, plus some passats have the PB engine fitted too.
for the 2.0 conversion you can use a carb head & both manifolds, only issue is the dizzy. need to get a dizzy adaptor ring to fit it, plus swap the gear off the bottom of the old dizzy. I'd fit a GTI head and k-jet GTI dizzy to get the best out of it, later PB one. Need drill the back of the head for the carby water chanel if you retain the 2e2, if you fit a webber no need to do this.
Id recommend getting a MK1 GTI/Rocco dual downpipe setup, the one with the 6 bolts as this will flow better than the toiletbowl setup on carby, rest of zaust should plug right into it.
Last issue is the extra block height, since the carby airbox sits right on top may have trouble with bonnet clearance, aftermarket filter may help but you'll loose the warm air feed without which makes a carb pretty unreliable! I have seen someone make a custom one out of a tupperware box however
