The service manual is vague about the type of oil seal used in the transmission. I would think that it is a standard type of oil seal but by the description, it might not be. You need to remove the chain, chain guard and the chain sprocket to get to the seal. At that point the service manual says "Remove the sprocket-chain spacer and the O-ring underneath by means of a screwdriver or a similar tool. Pay attention not to damage the bearing."
It sounds like the seal is just an O-ring that rides on the spacer. If that is the case, no wonder it leaks. At least it should be easy to find the O-ring somewhere (most likely metric) and pop it back in. If you see a conventional seal when you get the sprocket off, a hook type tool or a small pry tool can get between the lip of the seal and force the seal out of the housing bore. Even easier if you can get the spacer out of the way first. This will totally destroy the seal but there should be some numbers on it to identify it. If not, a caliper can get you the bore size and the spacer OD to get a replacement seal.
You don't want to get into further disassembly of the transmission as the shifter ratchet is under the cover and you don't want to have to re-time that thing. Also, the cover has a paper gasket under it that will be destroyed when removed. As we all know, you would be making one by hand as there are really no parts for these machines. Good luck and report back your findings.
