are you worried about the finish (i.e. its polished or whatever)? If not, you can buy a bolt that will screw into the snout of the pump and then use a nut & washer and tighten the nut slowly until the pulley is fully seated... Make sure you get as much bolt threads in there as you can, first. Maybe use two flat washers between the nut and pulley, with some grease or antiseize on them to create a sort of thrust bearing and reduce friction.
When you buy a new pump at the parts store, thats what they come with is a nut & bolt.
another idea is to heat the pulley up? Supposedly it will expand and slip on easily. Never tried it but its supposed to work well.
good luck