to get the pulley off spray the shaft with some PB blaster take a puller and pull the pulley off , if this wont work you could try heating the gear with  gas torch, since its aluminum this may make the pulley expand enough to remove.
To rebuild the pump after the pulley is off remove the 3 bolts and make sure you get the O rings that are under each washer, these should be replaced.
Refit the pulley so it can be held on with the nut and then gently pry forward on the pulley this should give it enough grip to slide the pump housing out of the block.
Remove the woodruff key and then push the shaft out the back (note that the outer gear and the center gear both have dots in their faces the outer gear MUST be installed the same way)
Once the shaft is out then tap out the front radial seal and remove the old housing O ring.
Clean all of the parts, tap in the front radial seal use oil for the shaft so it slides in, you can use oil for the O ring seal or I use Dow Corning 111 non setting silicone sealant so the O ring will stay pliable.
Refit the outer gear paying attention to the dots and also on the outer gear, it has a beveled outer edge thus must face to the front of the car or away from the engine block, the dots should be visible.
I suggest that you use STP oil treatment for assembling the pump gears , this helps it suck/seal when it is first turned.
Since your using the new steel gear you can remove the 1mm spacer that was behind the old gear then install the pump into the block paying attention to its orientation it can only fit one way but may have to be pushed in with the securing bolts. Also renew the bolt O rings