If there is nothing wrong with the rudder other than it not fitting the class rule of not extending below the bottom of the keel, I would just sail with it for now until you are comfortable with the boat. Nobody is going to kick you off the lake right now. Since Hickman no longer makes boats, you are going to have a tough time finding a rudder. Your best option is to modify the one you have or build a new one from scratch. There is at least one good step-by-step with photos in this forum. Just do a search for it. It has been a few years, so it is down in the stack somewhere.
My guess is the current rudder is a solid wood core. You can probably file down the bottom of the rudder and then but some glass over it and fair it smooth.
Once you start beating other skippers on the lake, that is when they will start to complain about your illegal rudder.