I ran into a similar problem with a rotten floor thanks to mice, ants or termites, and water leaks
. As can be seen in the following pictures.
I cut out beyond the rot and cut the top part of the fiberglass that held the panel to the exterior shell. I then scraped and sanded out the rest of the rotten flooring from the lower edge to keep a partial ledge for the new piece to attach to.
I attached "scab" pieces to the remainder of the floor to support the new floor as well.
Glue and screw in the the new floor piece and proceed with glassing in along all perimeters of the new floor panel.
Sorry I don't have a pic of the new panel or the glass work, I haven't been able to locate them. Guess I either didn't take any, or they got lost in the multitude of 1000+ pics that we've been taking during the rebuild.