My grandson received this instrument, used of course, as a gift and asked me to look it over for him. I'm guessing that it's seen it's share of gigs as it was so covered in nicotine that it appeared to be orange in color but once cleaned up it showed a nice mellowed ivory shade of its original white. It's a bit road worn and in need of a new nut but otherwise nicely set up and plays surprisingly decent. It has the hard tail bridge and soap bar style passive pups and with a 32.25 scale length by my measurement, a pretty basic bass guitar but I kind of like it. The tone pot is out of it also and I'm not sure what to replace it with so I could use a bit of input in that regard. Any additional information on these would be helpful as well.
The Yamaha Motion bass (including your B-MB-III) was a series of somewhat compact-sized basses made in the early to mid 1980s. Ibanez produced a very similar model that they called the Traveler Bass, though both of them are really full-sized instruments with a shorter scale length. The Motion B-MB-III had a rather unusual and complicated bridge assembly; the sort of thing you usually found on a headless bass. They were good, serviceable basses. John Entwistle of The Who seemed to enjoy his Ibanez version of that bass so much that he used it in an instructional video. I have seen the B-MB-III sell used in good condition for between US$400.00 and $500.00.

As for a replacement pot, there probably is nothing unusual about it. A 500K pot would probably do well, but take a look at the old one to see what its rating is. Pots are cheap, and easy to install.
"Maybe this world is another planet's hell?" - Aldous Huxley
Thanks FatalGear41. The volume pots are just a bit sticky as well so I may just go ahead and replace all three while I'm in there. Much appreciated.