My son's 125 developed an oil leak out of the left side, and I presumed it was a shifter seal leak until I started investigating further. I figured that the leak was caused by excessive crankcase pressure, so I added a valve cap vent. That seemed to free up some power, but the leak remained.
The photos below show the removal of the flywheel, stator plate and o-ring. The replacement o-ring, part number 91301035003 was about $7 at the local Honda dealer. No more leaks.
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