Drop the rudder and take it home to a warm area. Drill a number of holes in the part that is wet. Water should drip out. After a month or so the dripping stops. After another month or two inject epoxy into the rudder and then wrap the exterior with fine cloth and epoxy and then fair it out and paint it. This works for many other boats, and might work for you.
Total replacement is seldom needed. I presume the shaft is straight and the structure is solid.