I can say this much...starters don't know positive ground from negative. We convert old Ford tractors from 6V positive ground, to 12v negative ground all the time at the shop. We do absolutely nothing to the starter, and man does it whip that ol' girl over!! Since it doesn't have to crank very long at all, the starter lives just fine on 12v negative.

In my area there are electrical motor rebuilders that can repair almost any kind of electrical motor or generator. We get most of our rebuilt starters, generators, alternators, etc. from just such a rebuiler. (and we work on a BUNCH of old Ford tractors..) Check around your area for something similar.