I think your problem is the locking bolt diameter is too small, it's length too long and the material weak, specially if it's got no plain shank on the bolt.
I would drill the motor shaft and sprocket to a larger diameter, then tap a new thread to the motor shaft and use a proper bolt as already mentioned, high tensile or similar.
If you were not so much pressed with time I'd have sent you a couple of titanium bolts, it would be difficult to damage those :wink: