Denny, those engines usually have a straight output shaft with a keyway milled into it. Chances are the pump just slides over the engine shaft and it's stuck/rusted in place. Try driving a couple of thin chisels or cheap flat tip screwdrivers between the crankcase cover and the pump housing after applying a little persuasion with a Victor Hot Wrench to the center of the impeller. Is there a pipe plug or other opening that might give you access to the coupling? Maybe there's a set screw that bears down on the drive key.
Jerry

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