I have been doing fiberglass for many years on boats, fabrication and repairs. Polyester resin has it's limitations. It will stick to some plastics really well and others not so well. For the most part though, it's not reliable enough to say yes it is your answer. It sticks to PVC ok not great, polyethelene nope, polyurathane nope not really. It just depends on your pump housing.

The best suggestion I could give is the same as ChristopherS64. I normally use a heat gun and a nylon wire tie and a small putty knife/old kitchen knife, or a larger piece of similar plastic, and heat the case up till it gets soft and use the putty knife to weld/smooth the area over. If there is a hole to fill add the wire tie, or similar plastic and smooth it over. It sounds difficult but it is really simple and works great. Just don't burn the plastic, heat the plastic up slowly by aimming the gun at it for a second or 2 then back of for a blink of a eye, then do the same over and over till the putty knife/kitchen knife can start to move the surface around and it gets soft, keep heating it till you get it soft 1/8" in or so, and mix it around/ smear it together, add some more plastic if you need to.

I have fixed plastic valve covers on engines, plastic water holding tanks on boats, it works great and is relatively easy.

Harbor Freight sell a cheap-o plastic welding kit for around $30, I don't own it, but it should work great for this if you don't own a heat gun.