If you need a Porsche motor, the 914 is the way to go. There are sites with Type IV (VW Type IV is "almost" the same engine as the 914) conversion info; a Google search ought to turn some up, plus a look around on this BBS and the
http://www.shoptalkforums.com one should turn up info. As should a visit to Raby's site at
http://www.aircooledtechnology.com .
If you're looking for cheap power, though, a built Type I is hard to beat. You'll be building it over and over, but if you want to put a big shot of nitrous into the motor you'll be rebuilding just about anything frequently anyway.
--DD