You can make a suitable replacement for the factory plywood floors from tongue-and-groove wood of sufficient thickness (depends on the strength of the wood) pretty easily.
I used aluminum angle underneath to hold the boards togther, screwing up from below. I put some pictures on the forum a while ago, maybe they are still here. If not, I will repost.
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida