jetsled project boat

I have aquired an 18 foot jet fiberglass jet sled. I was given the boat as a project. The boat was a prototype, built by a highschool shop teacher in 1985. The stringers he used were fir 2X4's and the deck was plywood glassed over. The stringers were NOT left unglassed on the top and the plywood deck unglassed on the bottom. Webs that were put in are totally rotted out. The hull itself seems sound but the deck is rotten and the plywood delaminated. I cut all the deck outand am now in need of advice as to how to replace the stringers. A friend who worked for uniflite for 9 years in the 60's and early 70's had the idea to make the new deck out of foam and glass it on both sides. We are unsure if we can remove the stringers and do this and maintain the structural strength of the boat. Or do I put the stringer back in glass them correctly and use foam around them? Oh and weight is an issue, the boat has a 90 hp outboard jet, and as near as i can figure was about 4000 lbs in it waterlogged state when i aquired it. I am trying to keep as much weight out as possible. So far have removed about 600 lbs of water and wet wood.