I haven't seen a Jim Carter catalog in several years but was always impressed with the quality. So I guess he makes a 12v starter with the right gear for the 6v flywheel.
A car with an alternator might be able to push start if the battery was not completely dead. My pickup has and alternator. Or trucks have an amp meter, an alternator needs a volt meter.

"I wonder if God created man because he was disappointed in the monkey?" Mark Twain