It was common in the old days to rebuild pumps. Rebuild kits would be available with the bearing, seals, and sometimes impellor. You would press the impellor and hub off of the bearing shaft, replaces the bearing and seal, and press the impellor and hub back on. There was a certain distance that was to be held between the impellor and housing.

Today, you would be lucky to even find a kit, as everything is throw-away. There are still places that do rebuild water pumps, but that is usually for industrial and farm applications where the water pumps can be very unique and expensive to recast. Best to just buy new, or factory rebuilt if one is available.