This is cool:
http://www.renegadehybrids.com/911.htm It's a company selling all the conversion stuff to go V8. They include what extra has to be done to a 912. Looks expensive, wirh lots of custom work.
Does the 912 use the 901 type transaxle? They say its good to 300hp.
This guy is selling his for $16,500. Info at the bottom of this page.
http://www.912registry.org/tech/engineswaps.htm