Guys- it's a go kart, nothing more. I fail to see why we must overcomplicate the concept. Unless I missed something here, I get the sense that all the OP wants to accomplish is the ability to alternate between fuel and battery. So-
Live axle, on one side- classic kart stuff- gas engine as usual. On the other, a DC motor, maybe stripped off a scooter or something. Both require clutching, but what else is new?
Wanna burn fossils? Start the engine, engage the clutch, and go. Wanna go green? Engage that clutch, and the DC motor takes you there.
Without any unneeded fancy gimmicks like regen braking, where is all the complexity? This is a little highschool project, not high-dollar MIT venture.
I agree that it has the POTENTIAL to be very complex, but it need not be. I think we are getting lost in the "hybrid" lingo. To switch from hydro to dino, the OP may have to pull over, throw a lever or two, get out and start the engine. And maybe in electric mode it only does 10MPH and the battery only lasts 5 minutes. So? Why is this not acceptable?