If your leak is from the head gasket or the gasket between the cylinders and the lower case it is not difficult. If it actually is from between the case halves (less common) it gets a bit more involved. Suggest you thoroughly clean the engine and positively identify the source of the leak. Pictures are helpful.
It sounds like it may be the shift shaft seal or the countershaft seal. Both seals commonly wear and require replacement on these models. Pull the left cover And post a pic to begin with. If you see oil in there and can't see exactly where it's coming from, clean those areas and sprinkle some talcum powder (baby powder will do the job), run it and the goopy mess will be the seal that's causing it. Both of those seals can be replaced on the bike, without splitting the case.
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