I'm more familiar with large diesel EPA regs, but here's my take:
I don't think using an automotive engine will get you past the EPA regs. I think the regs are based on the year of the vehicle--not the year of the engine. In addition, I believe all passenger vehicles need to be chassis certified (not engine certified)--so the vehicle needs to be certified, not the engine.
That being said, here's 2 ideas for how you might get around the EPA regs if you decide to go commercial:
1) Look into what regs apply to "choppers". I mean, guys make custom motorcyles and trikes and I assume the EPA is okay with that.
2) If you're just selling it in Idaho, for use in Idaho, you might not have to deal with the EPA. This is because, it wouldn't be "interstate commerce", and therefore not under federal jurisdiction. I think I recall hearing about Montana or one of those western states that was using this argument with respect to federal gun laws. They claim is the guns are made in the sate, sold in the state, and marked to not leave the state, then the feds didn't have jurisdiction.
Just be glad you don't live in California!
http://abclocal.go.com/kabc/story?se...ent&id=4977150