I like to compare electrical voltage to water pressure. Most electrical devices are made to handle 20% above or below spec. A 6 volt starter will take the added pressure of a 12 volt battery, just don't crank it for a extended period of time! If you keep doing it, eventually it will not survive the added voltage.
Think most here will say With 12 v applied the current flow ( amps) will be half that of the 6 volt .........so the starter will be just fine if not better off with 12 volts.
