starter problems after conversion kit?

I have a 1947 Ford 8n tractor, I just converted it from 6 volt to 12 volt system with a conversion kit. For 2 months it started great. Today something in the starter broke it just spins does not engage. My question is; Did I need to get a 12 volt starter also ?Which I was told I did not have to. So I didn't. It is the origanal starter.