and your questions still not answered is it???
The "magneto cover" on the left hand,chain, gearshifter side??
The only seal between the magneto and internals is the crank seal and it would not leak gear oil from it,,
Are you sure your chains not slinging oil off or is it dripping off the gearshifter?
You can change those two seals (they would leak gear oil) with the bike still running just simply kick it over on its side and pull off the shifter or the sprocket dig out the old seal and hammer a new one in with something round,,,..........................
But a leaky seal is actually a bad bearing unless something screwed it up.