It will be far easier and far cheaper to buy a GTi, rather than make a 1.4 into a GTi, as you will need to replace most of the wiring, brakes, suspension, wheels, etc etc.
Insurance will also be a problem, as you are upgrading the engine, I owned a 1968 beetle which has a 1600 engine, instead of the 1500cc that it came out of the factory with, and most insurance firms would not insure the car without a full engineers report being carried out. And that was only 100cc!
So my advice is to sell the 1.4 and get a 8 or 16v MK3 GTi, and for me the difference in the insurance is very little, but I am 28 lol
Paul