I think the wheel base of the scooters is too short to fit everything in. You would have to put foot pegs on the front axle to get any foot forward position. A chain drive motorcycle would be much easier to fit to a motor.
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Weight has almost no effect on cruise range and efficiency where air is the enemy. But it is hard to pick up when it tips over.
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A used Honda Rebel would be easy to find a good price on and offer a low seat height and foot forward position.
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You could ride it on gas for a summer to make sure you like it before sinking any money into the project.
