if i were you i'd plumb it in as VW intended, if the rockr cover breather was enough for the 2.0 8v thats all it would have had! Plenty of 2.0 8vs to be found in the scrappy, was fitted to mk3 golf/vento, passats, SEATs etc etc.
Bigger zaust valves will help, but its alotta money to spend on an 8v, for the kinda money it costs you can get an ABF, which is pretty much a drop in replacement even down to the wiring since your car is a 90 spec!
Cam choice is a tough one, needs to be matched to the rest of the engine plus personal preference and I'm not really qualified to answer. If it was me I'd get whichever comes up cheap
The core plug might fit, best way to find out is to unbolt the breather housing and measure the hole, and compare that to the measurements of the core plugs round the back o the block.
