Rudders are not top flight rocket science. The main requirements are that they have a blade wide enough to deflect enough water, be strong enough to deflect that water without bending, and to reach down far enough to catch the water flow.
There are probably plenty of spares for other boats that can be made to fit. I don't know that market, not being into fast boats, but there are people out there who do have plenty of interest in making fast boats so there is bunch of suppliers of fittings like transom mounted rudders. Often, these are better engineered than the originals.