The pulley is cast so may well end up as scrap - but yes, my intention is to replace the pulley for a different diameter one. For the next few days I will just be adding penetrating oil in, to try and ensure rust is broken up as much as possible, then I will tackle attempting to remove it following all the helpful advice given here!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?