replace it with a round deckplate
there used to be some 3" round ones out there, but now it seems only 4" round are the ones I see on the shelves at marine suppliers. A hole saw will work but you would need something to hold the center over the hollow rudder post. It's real important to keep the rudder post from getting rain in it because it will go down in the rudder and cause internal water damage. good luck!
