I like Greg(Wolfgang's) idea of the Tribeca motor and a 5 speed. I happen to have a 5 speed, and also know where a Tribeca 6 with harness is located. That motor is only 2 inches longer than an EJ25, so AC in a 904 becomes do-able. The big intake in the 904 nose is tailor-made for a watercooled motor. 250 hp stock, cheap, and reliable. I'm sure it could be massaged a bit.
I really want to put that motor in SOMETHING! I don't think it matters anymore what motor you put in any replica.
Yeah, a 993 motor is awesome.