One of the best mounting adhesives I found is auto windshield mounting adhesive. It appears to stick to anything even plastic gas tanks just make sure the surface is clean. BTW if you get this on your hands it will have to wear off, once dry even acetone won't remove it. I have made engine mounts using it too. So if you have a crack in you fiberglass just fill it with this and even if the crack opens and closes this will act like a rubber patch. It costs about $7 a 14 oz caulk tube which will stick and fill quite a bit. I wouldn't recommend it for gaskets though, I used it on a VW rabbit valve cover and I swear you could have lifted the engine with the valve cover. The only way I found to remove the cover is heating with a propane torch.