I would give Tim at Mac 1 a call. He is the man when it comes to Benji's and Sheridans, as well as many other Airguns. You may be able to locate a bolt from an old parts gun, but I bet you could get a re-fabricated one from one of the guys like Tim. I have sent several of my Airguns, both vintage and new, to his shop and he does tip notch work.
Mac1 Ultimate Airgun Shopping