One easy way to repair both the
leaks and the sag is to remove the top and take the shell to a fiberglass shop that has a "Chopper Gun". They can shoot a whole new inside liner, up to about 1/2" thick, and also put some bow back into the roof. I have had cracked holding tanks encased in this manner. You might start with a local boat repair shop.
Short of that, you can remove the top and recoat the entire inside with resin, that will at least stop the
leaks. You can buy Bondo brand resin at most auto supply stores.