Please be aware that this is not a personal recommendation but I am told that those who convert Fench cars to 12V often retain the 6V starter.
I understand, the starting is as good – if not better – than with a 12V unit and, used “diligently”- i,e. not churmed ober endlessly, they appear not to suffer from the experience (- they being the starter motors, not the owners).
B…………….
Bernie