How can I replace an Armstrong Astro 20B pump?
From looking this pump over and viewing the PDF documents on their website, the one major installation directive is that it must be installed with the shaft in a horizontal position.



The Impeller housing can be installed in any position required by the plumbing installation by removing it from the pump motor and reassembling the pump with the terminal box on top so it's mounted at the highest point so water leakage will not intrude.

The motor and water seal are contained in the motor housing with the fiberglass reinforced polypropylene impeller being attached to the motor shaft. Some sort of epdm gasket is used between the pump body and the motor assembly.

As long as the pump body is free of corrosion, especially on the gasket surface and you can get the motor assembly and gasket from the manufacturer, what you propose should be feasible. The impeller is plastic and therefore not likely to damage the bronze or cast-iron pump body.

Being dry run for a couple hours would burn out the water seal and possibly, the motor bearings.