Rudy with your brakes is it possible to machine them out a bit and even if they are oversized, it is on a tractor and a company that bonds linings onto shoes will have an oversized thickness lining to bond on and that certainly will be good enough.
My auto electrician a while back had to reverse the polarity of a second hand regulator for me. He did it in less than a minute but I didn't see him do it. So ask your auto electrician to check it out and if you want to go negative earth do it all now. Reversing the polarity does not effect the rotation of the starter either.