It used a very common Delco-Remy starter generator. Easy to find and used on many other tractor brands. 750 was one of the few tube-frame Bolens with a aluminum 7 horse Briggs & Stratton engine. Briggs engine 170402 or 170401.
Starter generators were common during that time period because they can be easily added to rope-start engines.
If you don't need it to look original, you can also put a regular starter motor on it - instead of the starter generator - but that requies a different flywheel. It also leaves you with a starter but no charing system.