To "fix" badly deterriorated reel-to-reel tapes from about that era, we had to be bake those in a convection oven. That'd set the oxide so they could be played ONE TIME and copied. Several cuts from old reels that will be on my almost ready new recording were saved that way thanks to Rich Warren of WFMT Radio in Chicago and Tom Martin-Erickson of Wisconsin Public Radio. Not sure what to do with a cassette, but I suspect the tape could be removed, baked, recontructed and then copied.
Good luck !