I agree that "the final" option is cutting off the pulley. Carefully drilling the hub next to the keyway - radially out from the keyway ( maybe only 1 or 2 x 1/8" holes?) - after drilling the key with as large a hole as to think safe before risking damage to the shaft - should release stress in the hub when the hub cracks open. Thus enabling it to be pulled-off the undamaged shaft. If cast, the pulley will be scrap. It otherwise you may be able to recover it by welding (depends on your skill) with some finish machining. But I understand you want to fit a different pulley?
K2