Its not an easy task, you'll need all the engine loom, perhaps the fusebox & car loom as well if it doesnt have the same type of fusebox (pre-90 fusebox, hazard switch on dash, post-90 hazard on column) You'll need the fuel pumps & lines, zaust, driveshafts, gearbox etc. Ideally you want to uprate the brakes, and fit the front anti roll bar, as well as the rear beam, arb & disc brakes.
You could just throw the carb injection bits on the GTI lump and basically have a 1.8 carb lump but with a better head & cam. Problem is the GTI head doesnt have the coolant feed for the carb, so ideally you'll need a webber carb otherwise it will never run properly. Other option is to fit the carb head on the 1.8 bottom, or take the GTI head off and drill the coolant channel.
At the end of the day the carby has to be pretty mint to justify the effort of building a car you could just go out and buy!