There ahs been a thread a couple of years back about that, Jim. Not primarily about that, but it got discussed. It might have been the one about wilbyhillbilly's work (which although reasonably priced is not free!).

The problem (or part of the problem) is three-fold.

The magnetised ironoxide layer may have de-magnetised in whole or in part resulting in a "faded" or "muffled" recording. Also the magnetism inone layer may have affected teh magnetism on the next layer so that an echo or pre-echo is heard. This is called "print-through. DO NOT TRY IT TO FIND OUT WITHOUT READING ON.

The iron oxide that forms the magnetic layer may no longer be very attached to the tape. It may fall off on one playing. So whatever you do make sure that your first "pass" of the tape is recorded. It may be your only chance. Once it has fallen off, there is no cure. I believe there are some very expensive ways of treating tapes to reduce the risk (I'm sure I've seen methods) - but they may be affected by problem 3 below, since you have to unspool the tape to try to improve the oxide adhesion.

Finally the tape itself may have become brittle or too fragile to take normal playing tension. I don't think there is anything you can do about brittleness. It may be the case that you can reduce stress on fragile tapes by using a direct drive tape deck rather than a conventional capstan and pinchwheel one, but I wouldn't bank on it.

Csharp House may have experience. Or the British Museum.