The original letters were indeed heated on and when they start to come off, cannot easily be replaced. I would avoid clear nail polish if at all possible.

If I were going to do this, I would use something like Testors Model Paint Clear Coat (not the spray can kind, but rather the brush on kind.

You could then use what the call a modeler's Pipette which is a very small eyedropper like utensil that would allow you to drop tiny amounts in the depressed areas where the letters are. Once it all dries, you'd have possibly little clear humps over the letters.

Lyndel