Shimming works on helical cut gears, not straight cut ones like the ones in a Chinese 2 stroke. Helical gears have rounded surfaces that when properly spaced or positioned to each other relative to the curvature of the gear tooth will run quieter and more effeciently. Straight, or square cut gears mesh at the same angle so shimming either the drive or driven gear in or out will not have any effect on gear mesh only the amount of surface contact. Hence no noise reduction. This is an over simplication of gear engineering but I hope it answers your question.
Tom