Did this same job to an old Bluefin I used to own. (1996 I think, boat was a 1985)
Remove the old plywood carefully, it will become your template to cut out the new pieces.
My plywood was riveted to the stringers, I had to buy rivets from Fastenal, there was nothing at the local hardware store that was even close.
I bought the vinyl floor covering and glue from Bass Pro, they had an exact match for my boat.
Take your time and use stainless hardware to upgrade anything that wasn't originally stainless, mine came out great, better than new and it was a great experience. I had zero problems with it and sold it in 2015 to buy a bigger boat.