The motor and gearbox come together as an integral unit as shown on the left which is from the same manufacturer. I'm interested in those 2 set-screws. I'd think they would go on the top side and into the flat surface of the D-shaft. Did you take photos of that? I can't imagine why a builder would have used an odd-ball (or metric) screw size for something like this...in other words any standard key set ought to do the trick.