I believe that a 12 volt starter has a different number of teeth. It might crank on 6v but it probably wouldn't start very well(or last that long). Now if you're going to use a 12v battery and 6v starter that would be different. It seems to me that you are going to keep the 6 volt system(right?) and in that case as long as you clean all of the connections up(it wouldn't hurt to take the starter apart and clean it out unless it's trashed) your starter should crank the engine over as fast as it was designed to do(just make sure that the battery stays charged up).
