Measure the width across the two belt notches in the gearbox plate; the OD of the pulley is slightly less than the distance between the outside edges of each notch since the belt has to go though there. You'll need one with a fairly long hub because it hangs down pretty far below the gearbox. I think a 3/4" shaft uses a 3/16" key, but I can't remember, so hopefully someone will provide that measurement.
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