Seems unusual for the magnetism in a Kangol belt to weaken. My belts and all the others I've seen over the years have worked just fine.
Is it possible that there is something on the magnet or the other end that can be cleaned off? As a test, does the magnet pick up a bunch of paper clips?
Finally, maybe a little silly, but is it possible that the belt is not being mated correctly...maybe backwards or upside down?
To answer the original question, a small magnet can sometimes be revitalized by attaching it to a powerful, larger magnet for a few hours.
I don't think replacement magnets are available. Last (and expensive) solution is a new set of belts.
Richard M