From what I've read the conversion to the existing 1.8 block is normally either to 1.9L or 2.0L, firstly the standard bore is increased from 81mm to 82.5/83mm, and then to get to 2L you need a new crank for a longer stroke. I'm guessing now but generally if you choose to use a different bottom end the majority of advice seems to point to 2L short blocks i.e. Audi 3a, early mk3 2e, which I assume has both an increase bore and stroke and also has no clearance issues when fitting into the Mk2.
This is a conversion I'm thinking about having done later on, however it's costly as to get any decent power gains on the 8v you'll need a fair bit of head work in order for it to breathe properly...
