Much as I love old Porsche engines and especially the sound of a square port exhaust - I would avoid buying a replica with a real Porsche engine. 98% of time you don't see the engine - but you end up maintaining one! The cost of real Porsche parts and Porsche labor rates simply scares a lot of folks (as evidence by the large replica market).
Just trying to support idea of separating the valuable classic OEM engine from the replica. It makes it easier to value and sell the replica.