If you don't have a HIN, I'd strongly suggest you try to identify the boat. At some point in time that will be necessary if you add an engine and need to register the boat or title it. And, a rudder that is "close" may be off by enough to screw up your handling, so it pays to get it right.
Wood is good in that you can have a local woodworker make one up. Foss Foam in Florida is one of the few shops making new fiberglass rudders up. They stock a number--if you know the boat! And while I've heard some good comments about them, I also have heard less than stellar, so caveat emptor.