That model mossberg is super rare, at least in this part of the country.
Here is a exploded view. The old time parts diagrams were not like the modern ones. They figured the guy was actually a gunsmith, and did not need to see how things fit together in a picture.
I suspect the main spring is weak. I doubt one will be found, so a close imitation might have to be used. Or depending on how much its been shot, headspace problems might be appearing.