I won't be at Hampton Downs or Taupo until late Spring/Summer now.
I only have 1x15 tooth and 1x16 tooth front sprockets (both 520), so no, I do not have any spares.
I know a guy who is fully into karts so will ask him and get back to you about a suitable (14 tooth) 428 sprocket.

Somehow I think Marcus127 may have found your problem - Make sure the two carbs are balanced; the two throttle slides must be aligned with each other to give even throttle settings for both cylinders. As he says, there is a balance adjuster screw in between the two carbs.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...