yikes - didn't realize is was a sunken arrangement - You could fill it with
epoxy and fashion a new step plate from solid
aluminum and through bolt it and seal properly. You may need to cut a little from the base of the mast so the
rigging still fits unless it is not too much higher and the turnbuckles have enough extension capability. Or you could do as mickmul suggests - though I would put a small diameter through
hull and connect a hose where it exits under the deck that leads to the
bilge so water is not wetting any
interior surfaces - only if you can hide the tube or hose. Good luck