Floor Repair on 71 20' Seafari

[MarkJ]

How would I go about repairing the floor where the seats mount to? It is soft only where the seats mount to the floor. The screws that hold the seat pull out with rotted wood on the threads. So I was thinking about cutting just that section of the floor out and replacing the wood if possible or could I glass in a piece of wood on top of the deck just to hold the seats in place. The rest of the boat is solid, I replaced the transom about a year ago.
Also I am looking for new Power and was thinking about a 2002 Suzuki 90hp to replace a 72 Evenrude 125 electric shift. Is 90hp enough power? Any advice would be great, thanks