Well it depends. Which sprocket has the set screw and key? Is it the easily replaceable one or the one that's hard to come by? Next question is how tightly does the sprocket fit on the shaft?
If the sprocket in question is the easily replaceable one. And the fit of the shaft is SNUG. I would go with just a set screw to the flat of the shaft.
If the sprocket in question is the easily replaceable one. And the fit of the shaft is loose but less than 4 thousandths of an inch loose(.004 is about the thickness of standard 20 lb printer paper). I would just set screw to the flat side of the shaft.
If the sprocket in question is the easily replaceable one. And the fit of the shaft is loose like MORE than 4 thousandths of an inch loose(.004 is about the thickness of standard 20 lb printer paper. Then I would drill a small dimple opposite the flat part of the shaft. If no drill, you could even just file the shaft a bit flat, but not anywhere near as flat as the other side, just a little flat. Then carefully and tightly wrap the shaft very smoothly with aluminum foil until it's about .004 loose, and scotch tape to hold it from unraveling until you can gently slide the sprocket on.
If the sprocket in question is NOT the easily replaceable one. I would only do the set screw to the flat of the shaft. And only of the fit is snug as in you can feel it dragging the sides of the hole when you press it on the shaft. Anything else and I would probably not mess with it until you get the motor repaired.
