Custom/Hybrid fixing spongy fiberglass bottom?????

have a stunt hull, and there is about a 3"x3" square in the bottom that is spongy. how should i repair it? i can reinforce from the inside because there is a piece of plywood fiberglassed down as a fuel cell mount. could i just sand bottom and reinforce it? should i cut out the spongy area? or just go over top? thanks!
 
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