All depends on the money you got to spend... you can fit anything into any car if you have the cash to spend...
Just remember when you start upgrading your engine - you need to think about cooling, brakes and suspension upgrades as well..
Enlarging the bottom end to 1.9 or 2.0 is an option / rebuild head and put enlarged throttle body on... (120-140BHP)
Put a mk2 Golf or Carrodo G60 engine transplant (200BHP)
Mk2 Golf 1.8 16V transplant also - cheap and easy to do these days and well documented.. (150BHP)
Mk4 Golf / Audi TT - 20V engine transplant is the one we all want really....

(200+BHP)
I don't know all the details to do these... but there is loads of information on these on this forum and other VW forums...