It is most likely seized with old oil, if you have some water based de-greaser such as (purple power, krud cutter, etc..) and also wear gloves while doing this get an eye dropper put a drop or two on each side of the pulley and in the set screw hole let sit awhile and if that doesn't loosen it get a hair dryer and heat the pulley add a few more drops of de-greaser take a small hammer and tap gently each side of the pulley while holding the shaft of the motor to prevent to much movement inside the motor of the shaft, it will eventually come loose. I have used this on a couple of the old machines and it worked like a charm.