spare rudder
In your case, a transom hung rudder-like the old 25, is the right call. You might even locate a rudder/tiller from an old 23 or 25, install the gudgeons on your transom, and you are good to go! Hodo is right for bigger boats-although there is a difference between what is legal for some of these races and what will really work. We have used the dinette table (rudder) fastened to the spinnaker pole (tiller) on some boats-others have used the head door-you get the point..but if you want one that works, use a real one and test it. For your boat it should be fairly easy.
If all you need is a spare/emergency tiller, anY metal fabricator can make one up, or as Lauren says Pacific Seacraft may have them (for wheel steered boats).
Good luck!