I have been working on a similar project for the grandkids who dropped one off the trailer while towing. This is how I did it as I wasn't too concerned about it looking like original. Just wanted it the 1995 SeaDoo seaworthy.
First I watched some fiberglass repair videos on YouTube, just search "boatworkstoday fiberglass". This guy is very talented and has a tone of great videos.
I cleaned up damaged area, got rid of loose stuff, roughed it up with sandpaper in some areas, dremel tool on small spots. Used acetone as final prep cleaner. For the areas where hole went all the way through I used TotalBoat Polyester Structural Repair Putty as a base. Because the hole wan't real wide I used fiberglass drywall tape instead of glass mat for reinforcing. That Repair Putty has glass fibers in it also. That type of polyester resin it not really used as a finish layer because it stays tacky. The tackiness is great for the base though. You'll learn the different kinds on the videos. I went over that with PC-Products PC-11 Epoxy Adhesive Paste, Two-Part Marine Grade, 1lb in Two Cans, Off White 160114 as the final finishing epoxy. It is thicker but the best thing to use when working underneath the hull. That stuff drys to a super hard finish. Not the easiest thing to sand, so try to limit any excess. Better to go over it a couple of times to reduce sanding rather than one thick slab. I removed the engine and about everything else to work on the inside bottom first. I'm now working the same area from the outside. I'm not done yet, but when I get all the epoxy done I will use the black Rustoleum Hull Paint over the top of the damaged area to finish it. The Polyester Structural Repair Putty runs about $37 a quart, the PC Epoxy runs about $14 for one lb. I used one quart order of the Repair Putty and 2 orders of the PC Epoxy. No gelcoat used. So, with Acetone, etc, have about $75 in repairs for a pretty messed up hull. Looking at your pics, I'd guess you'll use about the same amount. Watch the videos before doing anything. Know the difference between polyester resins and epoxy resins. If you decide to use fairing compound, not all of them can be used underwater and fairing compound is not even close to being as hard as epoxy. I wouldn't use it on a hull myself. I didn't even think about using gelcoat for the bottom, wasn't needed for my application and I don't have the expertise. If looks are that important take it to a pro.