W O W
Ultimate iFixit Blast from the past, i commend you for not throwing this out.
when i was little i had a few walkmans in the early 90’s, and i’ll tell you all the things i did to make them work again.
Ok you replaced the belt, is there a way you can ensure it has a good amount of tension? Not too much though.
Then i would suggest making sure it’s getting the right amount of power, try different batteries. That was the problem 90% of the time.
It doesn’t sound like a tape read head issue, but a good cleaning will eliminate that.
with the age of this device, there is a possibility it is the motor, a drop of some good gun oil will free it if it’s guming up. Or you may look into finding a replacement small motor, albeit more involved.
Finally, the button can for sure be printed! This is a perfect project for a 3d printer.
you will need to do some measurements, and profile-it a bit, and likely prototype the suitable friction fit slot inside (assuming its like the ones i had, with a metal spigot - which you’re using now) - THEN Paint to match!
Please keep us updated on the fate of this little gem :)