It's typically easier to not grind or cut out all of the damaged glass to start with, the old damaged surface is used to duplicate the shape.
Use epoxy, most PWC hulls and bodies are made with fillers that compromise the bond when using other types of resin.
You can use something like cardboard to tape onto the outside as a mold surface to help get the correct shape back, then do the first glass work on the inside. Packing tape can be used on the cardboard so the epoxy doesn't stick to it. You don't need to put all the glass on the inside at one time, you can do few layers and then get started on the outside too. After that is hard you can do more of a cosmetic repair on the outside, only a small amount of glass will be used on the outside. You can add as much glass as needed to the inside.
No mat is used with epoxy, it will be a heavy cloth (10 oz) or something like 1700