I can tell you that the SB side has a poly'd plate, with a separate king plank - as said, they are usually rotted out. I had to replace all of that in 2010 in my first 25, and it was a nasty job.
What I did was use 1/4" ply with cloth between, building 3 layers to 3/4" or so. made a sandwich and bolted the whole thing to the hull WITH the motor mount in place, as the holes are slanted in the mount wedge, which offsets the screw holes a little. The screw holes were 1" poly plugs, holes cut, taped on one side, then filled in to harden, then drilled out to hold 1/4" screws for assembly, then the final 5/16th ones.
It took 16-20 hours to grind all that out of there, but it is now solid as a rock. Bad news is that the ex got the boat in the divorce, so I am now doing it again on another!
I would really check to see what the condition of that wood is, as that is where your motor sits, and the king plank holds your rudder. Either or these fail, and you are in BIG trouble.....if you need more detail on this let me know.
It's one if THE nastiest jobs I have ever done, but it is worth it. The grinding is endless.