What you might do is make one that's a bit oversized for the boat. You told me the boat only does around 20mph. That isn't fast, especially when compared to what I run. If the rudder was made of plastic, it was just a matter of time until it failed anyway so here's what I would do. Since the rudder seems to go through the boat's bottom, I would measure the inside of the tube. Get yourself a piece of stiff, straight rod as close to the tube as possible. Just remember, if you have to force it in, it's too big. Get a piece of flat metal, same type as the rod would be preferred and between 25 and 50% of the rod's thickness. Slot the end of the rod to the thickness of the metal plate and secure it by welding or with silver solder, depending on the material. When done, slide the shaft into the hull and add a rudder arm at the top. At this point, you're ready to throw it on the water. You may find the boat to be overly responsive, meaning you need to reduce the size of the rudder, easily done with a grinder or metal saw. Of course, when you are done testing, you will need to shape the metal a bit for hydrodynamic reasons, nothing a grinder or powered wire brush won't take care of