Starters and generators are 6v are fine on 12, just the voltage regulator would need to be changed. can jump a 6v system on 12 no problem, just don't try to use the lights or anything. if the charging system works then it'll run without a battery at all. Lots of people used to run 8 volt battery's in a 6v system to get a little extra punch in the winter. The coil likely won't care if its only for a short while, otherwise put a ballast resistor in line with it or use a 12v internal resistor coil.
Positive vs negative ground doesn't make any difference 6v to 12v
Jumped a few trucks and tractors with 12 and never had any issues. If it charge then don't leave the 12v batt hooked up, but if it has no generator/not outputting itll run.
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