Makes things easier for me. This will protect the wood and help resist future rot damage. Any good exterior grade plywood should do the job for you. Then paint the transom, and that's that for the transom. Sure, you could use something other than wood for the floor boards, but I'm not sure what it would be.
Wood is cheap and fast, which is why it always appears in glass boats.
Do inspect joints, especially where the transom joins the hull to be sure they are strong and secure. You can, if you wish, sand down the hull lightly and re-paint with just about anything, an epoxy paint being the best, a polyurethane next. Follow the prep advice on the paint cans. I'd say that an outboard of about 15 hp would move the boat along nicely. You might get away with something a little larger, but you start to risk overpowering the boat.