I’ve been thinking a lot about this gas tank. I think you’ve got nothing to lose by attempting a repair. It would be easier than fabbing a hidden metal tank. The tank is already damaged, replacements have got to be very expensive. I remember my first fiberglass experience, working in a fleet garage. They brought me an International Harvester truck with a fiberglass tilt front end that had been smashed by a forklift in a lumber yard. It had not only been crushed, it had large chunks missing. I asked my boss why they didn’t just buy another hood and he told me a new one cost $9000 back in the 70’s , at my wages back then, that translated to about 5 months worth of work. Haha!
In the end it only took me a couple of weeks, and with fresh paint, it looked like new. I wound up repairing three more the same way, plus the owners boat once.
You had mentioned you were thinking about cutting out the bottom of the tank, and that is just the way I’d go about it. If you look at the bottom it appears the tank was cast in two pieces and you can see where the edges were joined , here I marked it with a red line. If I were doing this repair I would cut about 1” inboard of the joint on or about where I have marked a green line. Once you have the tank opened up, you can finish removing all that bad sealer.
Clean everything as best you can, then I would sand the entire inside surface to provide a fresh surface to apply resin to. You could use a very aggressive sandpaper or even some sort of grinder to make quick work of it, because you’re going to go over all of it with fresh sealer. I was doing some research into fuel resistant resins and there are some promising products out there.
I would be tempted to, on the inside,
1. Clean
2. Sand
3. Fresh fuel resistant resin to the entire interior surface
4. Grind all around those fuel taps and reinforce area around them with additional glass mat and resin
On the underside of the tank,
1. Clean
2. Sand at least a 3” wide path along the cut line, so you could lay down a 3” wide glass repair
3. Around your fuel outlets and cracks, use a grinding stone and dig out the edges of those cracks, so you can fill with fresh glass and resin, build up heavier around those fuel taps, the nice part is all this is hidden, so no need to sand and paint. It’s all just glass repair.
I found this product on eBay, it looks promising,
https://www.ebay.com/itm/EPOXY-RESI...a:g:pQkAAOSwRYBZjgx7:sc:UPSGround!85387!US!-1
Here are some interesting articles,
https://www.fibreglast.com/product/the-fundamentals-of-fiberglass/Learning_Center
https://www.boatdesign.net/threads/which-resin-to-build-ethanol-safe-gasoline-tanks.44720/
http://www.boatus.com/seaworthy/fueltest.asp
I know this is all unsolicited advice, take it for what it’s worth, me filling idle time on an interesting puzzle.
In the end, your biggest cost would be your time. What do you have to lose?