[morzh]

What you could do, short of buying a new part, if this one is in fact operational, is to get a plastic bottle of some sort, kinda like they use in pharmacies to dispense drugs into, of such a diameter that would make it fit around the potentiometer snugly, or any similar diameter plastic tube with a bottom, cut of the bottom part such that it is the size of the lost cover (half-inch or so) and simply glue it over the pot. After all it is nothing more than a dist cover.

If, again, of course, the pot is in working condition which is easy to verify.