I addition to all that has been said. Twelve volt system to a six volt starter will work, However the twelve volts to a six volt starter is hard on the starter drive and possible ring gear. The twelve volt to six volt slams the starter drive into the ring gear causing more punsihment to the starter drive its self. One should be able to get by quite a while before problems development.
It is possible to take the starter to a good electrical starter & genarator shop and have it converted over to twelve volt. This will slow down & allow the starter drive to engage at a slower speed.
I have done this on a couple of Super C farmall tractors and worked out well.
