Sea Commander
A sharp tap with a small hammer, not a smack with a large hammer 😀
I am looking at mine as I write this and would not fancy trying to remove the whole side.
If there is fuel in the plywood it might not glue.
Why not try this. Get a piece of ply the same thickness (remove the two uprights as they are only glued in) place the ply on the inside and with a sharp pencil mark the arc above the window and the shape of the window, cut these out and sand until they mirror the original, now cut out the bad piece, sand the edges nice and square with sandpaper on a block. Now place the new piece of ply behind, mark where you have cut, cut the new ply just oversize and sand with a sandpaper block until you get a nice fit. Glue in place with wood glue, clamp a steel rule to the outside of the frame to make sure your new piece is flush, fill the joint with filler and sand until you can't see the joint. Glue the two uprights back in.
A bit of messing but it will make a good repair.
Alan