Seriously, ever heard of a feedback loop? oh course you have.... The same is true for the motor, a controlled comparative circuit feeding back the output from two motors with a resistive element meant to keep the circuit from over heating. Start the motor with the battery, turn on the comparator circuit, and you will make power. Prove me wrong.
Never mind that, simply use a .9 sized 4 stroke hobby motor. CDI controlled, electric start, $150
Then connect a 300Kv Brush less motor and Regen circuit, and voila, 10A of current @2000rpm.
The whole thing weighs less than 5lb, and fits into an 6" steel box, fan cooled. All and all I spent less than $200 for a portable charging system I can use WHILE I ride..... Or I can just pull over and let it charge.