It's likely that the vibrations you feel riding at maximum speed do not originate from the engine. Engine vibration tends to be less irritating and at high rpm. If your motorcycle vibrates heavy at speeds of 100 to 120km/h it's more likely that one or both wheels are out a balance. Check if the wheels are properly balanced.
eow.... got my FURIOUS WHITE KAWA 125..... I RUND ON HI-WAYS UP TO 120KPH.... BUT WAT I NOTICE IS THAT..... THE VIBRATION OF MY WHITE FURA WAS MUCH COMPARED TO OTHER MOTORCYCS I TESTED.... WHAT WILL BE ITS REMEDY????