If you put an 800HP+ motor in that car PLAN on spending $50K on the rest of the power train......at least................1000? ....you will wish you didn't. And you will have a $300K car you can't sell.
I seriously considered using a complete Cayman donor car to outfit a 917. The Cayman R's have 340HP and a PDK gearbox. IMHO that's PERFECT for a street-driven car and if you ran slicks it would be a very low lap time track car. Why a donor car? Because it is GOING TO BE A SYSTEMS PROJECT and I would want a running car and a working gearbox. I would have wanted a car with the nose knocked off.
However, it could be a cost savings way to go instead of a V8/adapted gearbox power train. Here are some examples of donor cars for sale. I think $20K would do it and that's less than an LS/Porsche power-trained modified to work in a 917.
And imagine taking that car to a Porsche club track day. Hell, you would get offers to buy it all day long.
Here are some current Cayman donor cars for sale. Note: don't buy a flooded/fire car.