Here are my 2 project Walkmans I just bought...

WM-20
WM-10

The WM-20 came without a battery door. If I can't find a replacement, I will just tell the seller to take it back. Missing parts is unacceptable.

The WM-10 is totally fixed except I suspect the belt is a misfit replacement. It is too tight, and pulls the center wheel off-tilt, causing it to scrape against the outer case. This prevents spinning. So, I will also need to find a replacement belt.