well if it isn't "spooge" (the common term for unburnt 2 stroke oil)
you'll have to take the clutch cover, the clutch,crankshaft gears and collar off and use a seal remover to pop the old seal out then grease the inside and out side of the new seal and push it in squarely (may have to use a piece of pvc pipe that fits the outside of the seal and hammer it in squarely) Not too far though, if it goes in too far it will block off the passage in the crankcase that feeds lube past the main bearing.
then if the collar is worn a groove in it you could flip it over and insert it in the opposite way it was originally so the seal rides on an unworn surface,,, then just put everything back the way it was and don't forget to line up the dots on the counterballance gear and the crank gear.
tip: if you don't have an air impact wrench for the clutch nut and the crank shaft nut you can insert a penny between the small crank gear and the clutch gear to hold stuff still while wrenching on the nuts. do not use steel, nickles, dimes or quarters...only pennys as they are soft enough to not damage anything.