1. Bolts are at the hardware store. Nothing special about old truck bolts. Bypassing brackets because they have no bolts is a mistake.
2. If the end plate on the generator is not pot metal, take it off and weld ear/search ebay for a parts gen/place a wanted ad here in our classifieds for gen parts and brackets. I'll bet someone has an end plate.
3. Why no pics of your generator. So we can see if it's fixable.
4. There will be several years of gens that can be donors. You show a gen with a commutator band. That is up to about 1953. I am guessing that the end plate is the same for long and short.
5. I assume you have decided not to go with an alternator and that you have a voltage regulator and you are in the 6V club.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn