Best way to do it is find an engine with loom that matches the wiring spec of your car. Post 1990 the electrics were changed to CE2, easiest way to tell is to check the hazard switch, if on the dash its CE1 if on the column its CE2.
But don't bother building something you can buy, fit a 2.0 bottom end while you're there, or better yet a 2.0 16v ABF

Unless you REALY love your car its not worth it, as theres a whole load of stuff that the GTI has over the 1.6, plus insurance will be cheaper for a std GTI than a modified 1.6!
P.S. Hienz, the electrics changed in 1990 on MK2s
