Can I put a sprocket made for a keyed shaft on a D bore?

Depending on long the "temporary" motor has to be in operation, a set screw into the flat on the shaft will work fine. Be sure to use a pointed setscrew, not a flat one, and use a drill bit to put a divot in the shaft where the setscrew will hit the shaft. Lastly use some blue Locktite on the setscrew, or use a double setscrew if there's enough room. Check frequently until replaced.