If the starter is in good condition it won't care, just turn faster. Like any starter, prolonged cranking will make it veeerrrrry unhappy.

Find a shop in your area that rebuilds starters and generator/alternators, usually listed under Automotive Electric or Electrical repair. They can convert the gen to 12v if they're competent. You give up the charging at idle you'd get from an alternator, but you won't have to find brackets. However, if you search the web for the Buick club (s) they probably have a tech article or lead on someone who makes brackets for the conversion. Don't forget about the dash/interior lights, as well as the head and tail lights.