Anything is repairable. Without looking at the damage, it's a bit hard for us on the net to give an accurate answer. Is there someone GOOD in Perth that can weld Alum. cylinder heads? Depending on how severe the cracks are, they head my have to be re-heat treated to bring the hardness back up. I'd suggest you replace the WP, inspect the timing case for corrosion, replace the top & bottom thermostat housings as well as the thermostat when the head is refitted.