Don't know about performance but, the B's have the new crankcase with a different prop drive gear train for lighter weight. Real reason is simpler and cheaper manufacturing. Below are photos of the new and the old.
Thanks Milton. so maybe the cam is different? or is still interchangeable.
I would assume that is exactly the thing you couldn't change on them. But, I could be wrong. They may be interchangeable. Techs at Horizon would know for sure. There were also compression changes made to the motors to accommodate the Americans lust for nitro. Saito was selling motors over here like gangbusters, and they had to take care of this market.