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The pulley is press fit onto the shaft. The service manual doesn't give a procedure for removing just the pulley. It shows disassembling the entire pump to remove the pulley and shaft as a unit.
You can try removing just the pulley with a power steering pump pulley puller/installer tool that is usually a free rental from an auto parts store. The new pulley will need to be pressed on after using the installation tool.
That's what I was afraid of. Maybe try a machine shop to see if they can do it. The Lexus replacement pumps come with a new pulley already installed. Rebuilt ones don't so there must be a way to remove them. You could heat up the pulley with a torch to expand it and make it easier to remove. If that works put the new pump in the freezer for a few hours and then try to install the pulley that has been heated.