If your bearings are not burnt out, try cleaning the drive belt pulley's with some solvent.
If the pulley's are glazed from the belt slipping, give them a quick going over with some 220 emery paper, so the pulley "grips" the belt better.
You will probably need a new belt, if the original has been burnt-the carbonized rubber will never hold the pulley properly, after this happens.
If the bearings are shot, you should be able to source replacement bearings from an industrial supply outlet.
The bearing numbers may still be evident on the bearing itself, if they haven't burnt really bad.
Should the bearings have not had seals (they should have) try to find a replacement with such a seal fitted to it.
Best of luck with the repairs/refurbishment of your sander.
R.W.