The rotor doesn't come off the crank for this, It's easy.
Drain the oil, disconnect the battery, remove vent hose, disconnect the wires from the alternator at the plug, loosten the foot peg and let it hang out of the way, G5 riders pull the kick arm off, remove the 11 bolts holding the side cover on, pull the cover off, the gear is super easy to change, just remove the C clip and slide it off the shaft. make sure you put the new gear in the right way.
The seal is in the side cover, remove the snap ring pop out the seal, pop in the new one and put it back together with new oil and filter. It might take 45 minutes. I didn't even hurt the gasket so now I have a spare