I got a couple of busted Walkmans. Both have drivetrain problems, so the gears that turn the tapes are frozen. Usually this means new belts or whatever to fix it.
One day I was doing some research on repairing tape cassette players and ended up finding this on ebay...
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What it does is play music on an SD card through your player, very much like those cassette adapters that have a wire coming out of them...but wireless. And since it is not a tape, it does not need the gears to turn at all. You control the music via remote control.
It worked perfectly on both my walkman. They sounded great.
It is also a standalone MP3 player, you can plug in headphones directly into it and it has physical controls right on it. It has decent volume and sound. It recharges via USB cable, no battery swapping. I saw a neat mod where someone drilled a hole in a cassette tape case to protect the unit while playing it this way.
It does have a finnicky on/off switch. I had to insert the tape into one of the walkman in a very particular way to ensure the switch was not bumped off.
So if you got an old walkman that seems to work ok, but no gears turning. You might want to try one of these. It was only about $15 on ebay.
Search for 'telecontrol cassette' to find it.