tal911, simple question, why?
If you want a tt, sell your car and buy one, it would be cheaper.
A 90 C4 may not be the most desirable Porsche on the planet and having a tt motor in it wouldn't help. Good way to turn a large amount of money into a small amount.
Another alternative would be a supercharger. One of the local de fellows has an early C4. Had the motor rebuilt to 3.8, blew it up, put a 3.6 back, and then had Turbo Performance Center (TPC) in Jessup, MD 410 799 7223 install their supercharger kit. Looks like a factory installation.