Anschutz SuperAir 2001 repair

A family came to our club asking for help to "rehome" the collection of an older gentleman who passed away and had a great collection of target airguns and firearms. I'm already on the hook with this SuperAir 2001 (and a 1912 Chilean Steyr rifle), the only problem is that the sliding breech part is stuck and won't close, cocking the side lever or applying safety doesn't seem to unlock it. Any pointers on how to get this beauty in working order again? I promised to pay $350 for it if we in the club manage to get it firing again. Else we (or they) will be taking it to a dedicated air-gunsmith and who knows how much that's going to cost.