I am guessing that you are leaking from the main shift coming from the transmission. If so there is a main shaft oil seal part # 37741-82 and then there is according to the parts book a seal that belongs to the primary housing that has a part number of 12018. The oil seal for the main shaft does not require that the transmission is taken off the bike it can be replaced very simply with a screw driver however, always use a rubber mallet and a piece of pipe the same diameter as the seal.
I always find that sometimes the bike has some 1984 parts. I believe this has to do with the month the bike was built at least in my case. So, the primary housing might have a part number of 12052. 
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