I put a Corvair 6 cylinder engine in my Karmann Ghia many years ago. It just needed a reverse cam (since the corvair engine ran backwards) and an adapter plate. It took minimal trimming at the back of the engine compartment to get it in there. It was easy since the Corvair was an air-cooled boxer like the VW.

I think it was about 120 HP, but you could get a Corvair Spyder engine with a turbo and it would get you 180 HP. It was scary!