It will take very little effort to add a layer of 10 oz glass to the sole. You can run it up onto the sides a couple of inches then glass the sides over that to give a completely water tight cockpit. Glassing the sole will also improve dimensional stability to limit cracking around the edges. I would also epoxy saturate and then glass all the cutouts you make in the sole plywood to limit the possibility of water getting into the ply end grain. The fir used in plywood these days is nothing like what was available when your boat was built. It will rot if it gets wet.