I think it is a foam core. The person who would know best is Rudy at D&R Marine in Assonet, Mass. The phone # there is 508 644-3001. Rudy used to work for Oday, and has the molds for some of the Oday boats and for many of the parts. He sells replacement rudders and all of the hardware:
DR Marine Product List
I have bought a few things from Rudy for my '79 Oday 23; he is a very knowledgeable guy about all things Oday. Even if you don't want to buy a replacement rudder from him, he can give you some advice about how to go about making your rudder like it was from the factory.