I might be tempted to sand the surrounding
hull well to get a key, fill the hole with car body filler (polyester) to build up and then glass over it with csm and polyester resin making sure the edges are well bonded. ... that should do a fine job. in my experience, polyester filler is used a lot in fibreglass boatbuilding... and most importantly....relax! it's plastic! it won't
rot or rust like timber or
steel... also. if you want to build up the area around the stern gland a bit you could give yourself a bit more meat to bolt to....