I would spend the time and cut out that section of the floor and put in a new piece of marine ply rather than glassing something on the top. If you take that route you still have the soft deck underneath. At the very least you could over drill the screw holes and remove as much rot as you can then fill with epoxy and cabosil or glass fibers.
As for the power I would guess way to small. I have a 20 SF with a 175 and the consensus here seems to be 115 about the lowest you can go for a 20. I am sure others will reply on the power.
Welcome to the site. Hell of a first post and project.
Pictures always help.