Rotten wood in Stringers

[Hoover]

I got the floor up and most of the inner transom removed. When I cut through the stringers to be able to fit the new transom in I found that the plywood inside the stringers is rotten. The fiberglass surrounding the stringers is in good shape and is reasonably thick too. Do I need to replace the stringers or since the fiberglass is good can I leave them alone.