Any luck with the repairs?
As a last resort I'd suggest you ditch the reverse thread screw you're using, its too small at 1/4 diameter. A bigger diameter bolt will be much stronger. All you have to do is drill the sprocket, and drill shaft to a bigger diameter, say 5/16 (or 8mm) Buy an M8 or 5/16 24UNF taps and cut a thread in the drill shaft. Then lock all together using an M8 or 5/16 high tensile bolt. Then use a 3mm roll pin to cotter all together.
Can you post a pic of the problem area and the parts in question?