That's why things like "off center" need to be explained. We can't see it from our house. It's not off center. So I guess it's bend. But don't know what you see or when you see it. Take off pulley and check it on a flat bench. Put a bolt and nut in the center hole and spin it on a drill.
You asked "how do I get it centered on the shaft". You now know that it is centered via the tight hole. The pulley or the new pump is bent or spaced wrong. But again we don't know what you see? Wobble when motor running? Belt not straight....like crooked/angled pulley ? Belt not straight.....like offset pulley? Pulley rotating in an ellipse? You can put the pulley on without the fan to watch it with motor running. May need temporary short bolts.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn