Personally, I think the Honda GC series motors would be the way to go. They use an upright cylinder, a front exhaust, and a rear carb, just like a simplex.
This one used a chain to a jackshaft, and they to a belt final drive. If I were to build one, I would go with a belt drive clutch to the jackshaft. I bet this could also be done with a torque convertor, and all belt drive.