Your title is repair, refinish, restore. I would say those options if possible would be cost prohibitive given the age of the tub. If the tub runs and heats and functions as it should and you want to keep using it as long as you can and you want to improve the looks at a reasonable modest cost. I wouldn’t rule out “cover up as an option”. I have never done this to a hot tub but I have worked on old tractors with fiberglass parts and cars with fiberglass body parts and both are subjected to a lot of abuse afterwards and they have held up. Seeing as how most of your damage is outside of the water you could paint the surface after prep with some of the more modern durable paints. Epoxy types I feel would be worth a try and not break the bank with an old tub. It would require sanding to provide a tooth and a lot of careful masking to keep the overspray out of the tub and off the outside. Low cost, fair amount of labor, no guarantees as to how well it will hold up, and a slightly different look when done. If it was mine I would try the filter lid or an area along the back side as a test.
Just an idea.
