Sealings, Nothing more than Sealings.
I would try calling TA performance or Poston. Or even D+D machine the guys that specialize in specialty parts for these engines. If anybody knows of a crossover part or even the actual part you need it'll be one of those places.
Does the 300 have a rope or a neoprene type seal in it? If they all originally had ropes there might not have ever been a repair sleeve offered for them. Ropes usually won't wear the hub like rubber seal will. Does your balancer hub have a knurled pattern on it from the factory? The guys at D+D say on their website that they have tons of used parts and their prices look mighty cheap to me. Maybe they can sell ya a used balancer fairly economically. Good Luck.