There is a clutch in the transmission that has an arm that kicks out when you hit a certain RPM, you have to adjust the set screw till you get it to the tension that lets the arm kick out in the proper RPM range. Also about the Spur gear stripping you should replace your clutch bell with an updated version to avoid the stripping of the gears. The aluminum clutch bells get hot running against the aluminum clutch shoes and warp causeing the clutch bell to be out of round and strip out the transmission gears.

This should be the gear you need for the main spur gear:
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Clutch Bell and Clutch Bell Gear Upgrade:
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