The big issue is getting the one off the old motor you want to re-use. I would cut the crankshaft above the top pulley with a grinder or whatever you can get in there, then soak it with penetrene or a 50/50 acetone/atf, leave it for a day or so then see if you can put it in a press to break it free. If you go this way make sure when you put it in the press you support the pulley in close to the shaft and not on the face of the pulley because it will just bend the face of it
