How do I restore the original finish of a reglazed plastic tub?
I seriously doubt you will be able to return the original finish. The glaze you refer to was probably not applied properly or was cheap plastic paint.  In order to refinish a fiberglass tub unit, it will have to be stripped, sanded and properly sprayed out with a multi-stage epoxy refinishing product. this is a very difficult job and not one that comes out well when done by an inexperienced installer. The average cost in this area to do what you want is around $500 for a good refinish job. A half decent new tub and surround is about the same price without install costs. I have used a lot of Kohler 4 piece units, and been very satisfied.