That cover is a bearing keeper to stop the mainbearing walking out along the shaft. There is no easy way that I know of to replace the seal other than pulling the cases apart. A service guide is a very good idea.
I'm still not convinced completely that it's a seal problem. What does the plug l;ook like when you pull it out? Is it sooty black, or tar oil black? What grade of plug are you using? I have seen a faulty coil, lead or cap do the same thing. Maybe the carb float is too high? And as someone said, the generation coils at the flywheel may be partially shorting, lowering the spark voltage. An auto electrician should be able to test the spark voltage.
Nigel in NZ
The mouth of a perfectly contented man is filled with .
-- 2200 BC Egyptian inscription