Now that my interests are expanding to retrocomputing, I feel the need to read those C64 tapes. Which means that, somewhere, a tape/cassette player needs to be fixed.
I began from my walkman Sony WM-B12. A rather cheap unit back then. I suspected the failure was in the rubber belts. That model is just two plastic shells locked together that come apart with the usual gently-pry-here-and-there method. The remnants of the two belts were just enough to suggest their original path. They melted into a sticky dirty goo that is non-conductive. Hint: the goo is quite very filthy, so work on a disposable surface, use gloves and protect your clothes. Seriously.
The goo came away with mechanical method, so I didn't need to use chemicals. Then I needed to understand which belts I needed: square, round or flat? The remnants suggested square about 1mm per side. Maybe less, but I needed a proof it can work, I don't need perfect sound (from a tape?).
The usual Chinese sources sell a bag of 1mm square belts of assorted lengths. They came faster than expected.
I did not take any measurement. I just picked two belts that seemed right. And they were indeed! Now I have a working walkman and something to show to teenagers. Their reasoning about how we built a "playlist" back then is very interesting. Especially when they realize there were no commercials.
I was not that lucky with a later model from Aiwa with autoreverse plus AM/FM radio. It needs a longer belt, probably round and thinner. I tried the 1mm square and the result is very obviously wrong. Another time, maybe.
If you still have tapes and a player do consider trying a repair. It's pretty easy. Just be prepared to have a wayback machine in your ears when you succeed.
