jerni21,
Would a gas motor fit in the 'Darby'? I'm sure one would, but there are some things you need to think about. I would think the 'biggy' would be finding a motor that would provide the required torque since you don't really need speed. Or, gearing the thing down. Hmm, that'ed take care of some of the required ballast too. [I think there would be a lot of design work go into it and I'm way too lazy! I'd really like to see it done though.]
You're already aware of the problems associated with the motor breathing and being 'cool'...
- 'Doc

PS - Take lots of pictures. I want to see how you did it!