I had that thought too. The .40 jug is pretty small. It may have to be a 3 valve head with the glow plug where the 4th would normally be. 2 small intake, 1 large exhaust. I'm pretty sure I would want it to be OHC too.

It is more of a backup plan anyway. Thanks for all the cool links! If nothing else, I will have seen all kinds of really neat engines because of this project.