Doing some more reading and research, it looks like high-compression engines have all sorts of advantages of standard gasoline engines.
It looks like many propane kits are designed to go on DIESEL engines, and straight alcohol runs best at higher compression as well.
This makes me want to lean towards a diesel engine, as it could potentially run bio-diesel, waste vegetable oil, alcohol, propane, or natural gas.
POSSIBLY, it could even be modified to be a true multi-fuel engine, although I have no idea exactly what that would take.
I did just see a 1983 VW Rabbit for sale on Criagslist (1.6l diesel indirect inject engine) but the guy sold it before ever actually giving me any contact info!!!
As far as I know, there is still those two guys selling the two diesel VW Golfs and 3 engines, but that would be a BIG PROJECT!
One guy I talked to commented about modifying a diesel to run on alcohol, but it sounded like a lot of work. After reading about, and understanding the idea a bit more, it doesn't sound so far-fetched.
A truely clean-burning, high fuel economy engine, in a compact car frame? Maybe I should rethink my earlier
rant about the X-prize......... (Other than I still don't have the $5000 to enter!!!)