Swap in a 3.3 or 3.5 liter M20 "Big Six". Look for wrecked 5 series, 6 series, and 7 series....hey, I bet there are lots of junked E23 (78-87) 5 series Bimmers in yards or for parting out. Take a look at that avenue too.

It's possible to get 280 hp out of a normally aspirated SOHC 3.5 liter....that's almost 1990s M5 figures. Think about that....

-Sam