When I rebuilt mine I reused dads old stringers, probably 4x6 wood. I think dad rebuilt on them when I was a kid. We put a layer of old oil on them, a piece of tin flashing over, and nail or screw the true 1 inch cross planks over them. Mine is probably 16 feet long and 7.5 feet wide. Can hold 120 or so small squares, that’s about all the running gear is up to.
Anyhow, the metal flashing sure seems to do a lot to save those stringers.