Tower doesn't list brushes for their present 12V starter. That's bad news for your older one. Finding replacement brushes is going to be difficult.

No real way to tell for sure if it'll take higher voltage other than trying it. You might try a 4 cell LiPo first if you decide that higher voltage is worth trying.

I have replaced the brushes in a Tower starter many, many years ago. I was surprised they don't list them now.