I can certainly see the benefit of using a small gas or diesel motor. Once you have one, I'm not sure it's worth it to carry around the batteries and electric motor. You lose a lot of energy if you charge batteries and then use them to power an electric motor, versus linking your diesel motor to the wheels. It's something like half as efficient, after you've lost some in the batteries (charging and discharging) and some in the alternator and in the electric motor.