I was thinking of an "electric assist" really, not anything that can move the vehicle on its own, just displace some gasoline use, like you are trying to do with hydrogen.
You already have batteries and can probably find a suitable motor for < $100. Still a lot of details to sort out, but you want to make the most out of the batteries you have since you can't afford ones that only hold $2 worth of charge but still want more range.
I'm envisioning the frame and wheel off one of those little scooters with your motor (or maybe the built in motor) strapped on with appropriate gearing for your situation, leaning on the bumper.
Anyway, it is all pretty half baked right here so have fun
