Webco still had heads for YZ/IT 250, 400 etc in the 79/80 catalogue that i have. In the 80-85 period DG made a few heads for the last of the big bore air cooled bikes RM500, CR480, YZ490 etc although i have never seen one in real life i do have the pics/sales ads. They also made them for CR250 red devil but i have not seen a listing for YZ250.
If you think you need more cooling you could get creative and make your own fancy finned head or make a water cooled head.