Great bass, simple as all get-out to work on.
1st, try spraying ?pot cleaner and lubricator? into the pot opening with one quick shot, then work the knob around 50 times. Don?t use just pot cleaner, as it will tend to dry out the pot and make it worse.
Failing that to work, I would look for some vintage pots (CTS, Centralab?) 250k pots to replace the originals (Ebay) or just new ones if it is going to be a player. If you have any soldering skills at all, you can do it.
I actually have a vintage Centralab 500K pot dated 1964, but I think you need 250K ones for that bass.
I also would take a 9mm brass pistol bore brush and dip it in rubbing alcohol and shake it off and then insert it into the output jack a little to clean it.
Good luck,
Dave