This happend to me and I too changed the big oil seal only to find the problem returned.
It turned out to be an O ring on the shaft behind the crankcase cover. I did all the work with the motor still in the frame and only had to remove the cover behind the clutch to get to the 50p O ring. My incompetence cost me a new a new gasket as the kickstart assembly comes off too.
No problems since and I think this O ring failure is rare.
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if it had a tail - it would wag