The engine is easy enough to obtain, but the ZF transaxle is another story. Figure on $8K for a new one, maybe half that amount if you luck into finding a Pantera that's being parted-out.
There is allegedly a Renault transaxle which will hold up behind a Ford small-block in the GT kit car, but I don't think I could bring myself to use a Renault part in a GT-40 replica. It just seems so... wrong.