This 1950 Skil 525 chain-driven belt sander was running a slow, probably due to the hardened grease around the gears. These are also known as "locomotive" sanders.
I completely tore it down except for the area where the worm-drive is. I didn't want to risk breaking anything trying to access that area. I ended up just adding new grease to the worm-drive gears. Power cord should probably be replaced, but I wanted to keep all the parts original as this tool is going to AvE.
The sander takes a 2 1/4" belt, which is not very common currently.
The sander works fine now and it is back to near original condition.
Interestingly, this sander was ~$65 in 1950, which would make the cost around $650 currently!