Help with a 1967 Gulfstream Fury

[Hec In Omaha]

I need some help regarding my 1967 Gulfstream Fury. This boat is 18 feet long and has an inline Chevy 6 cylinder 250 cu. in. engine. The engine simply purrs. I did a semi restoration back in 2000. The boat was yellow and avocado green. I re gelcoated the entire boat to what you see in the pics. The interior was also completely redone. The only thing I didn't get into was the floor and what lays beneath. Now I have problem. I had a softspot on the floor just infront of the engine so I pulled the seats and carpet out and proceeded to cut a 2 foot by 2 foot inspection hole. What I found was saturated floatation foam. In addition the two 2x4 stringers on either side of the center stringer are completely rotten. They resemble a stacked deck of cards as they are now layers of rotten wood. So I decided to open the hole to about 3 foot by 5 foot to get a better look. I removed as much of the saturated foam floatation as I could reach.

The question is what to due with rotten stringers. I realize replacment is the best fix. Is it possible to replace the stringers with the engine in place? I assume that these smaller stringers run the all the way back to the transome. None of the 2 x 4 stringers have fiberglass on them. I can push them side to side as they are quite flexible and do not appear to be fastened to the hull of the boat. Should they be fastened to the hull? I suspect they weren't fastened to the hull to prevent point loading on the fiberglass.

Any help is appreciated.

[Hec In Omaha]

I dug out more floatation foam around the smaller stringers mentioned in my first post. These stringers run from under the engine to about where the front seats were mounted just behind the steering wheel. Since they weren't attached to the transom or hull for that matter, are they truly structural? On the plus side they are going to be easy to replace.