Don't do the tank in the stern. Fix the tanks, the job isin't as bad as you think. You can cut the deck out just above the tanks, cut the tops of the tanks off and grind the compromised material out of the inside of the tank. Depending on how thick it still is after grinding you might need to reglass the inside of the tank. Finally coat it with epoxy.
Or remove the fiberglass tanks and replace with aluminum.
There are many 31 Bertrams which had to have their fiberglass tanks either repaired or replaced. If you search around 31 Bertram tank repair I'm sure you'll come across an idea of what you're in for (albeit on a much smaller scale)
I concur to it correctly or your vessel will have zero resale value. I would call Fortier for the exact location of the fuel tanks. I am sure they will give you the exact location of cutting the deck or maybe even a pattern. Once the deck is cut out with a circular saw & blade set at 1 ". you will get to the tanks. The piece of deck you remove can be reglassed by stepping the edges.
Fixing the old tanks may be an issue. If you remove the top you will have a basin to drop in new aluminum tanks. Your new tank will be slightly small but not enough to make a difference. We used this method on a 28 Bertram with great success. The key is to have a good fit on the new tanks going into the old tank basin.
Once done screw wooden cleats under the deck around the deck opening. You will need to screw from the top. Counter sink the heads as you will be glassing over them. Apply and good epoxy like west system or hull & deck puddy onto the wooden cleats. Next screw the removed deck back down. Once again counter sink the screws.
Buy a roll of 4 wide biaxle fiberglass. Glass the step area on the deck with two layers of the 4" glass. You will have to remove about 1/2 of the top skin of original deck glass. Once done refinish with glecote. You will lose some of the factory non-side, but other than that you should be good to go.
This may sound like a lot but you can do it.
