Empirical evidence has indicated over filling of oil causes crankshaft seal to be pushed out.
Level is checked with bike standing strait up with dipstick setting on top of threads not screwed in..
35X52X7 oil seal can be purchased through any automotive parts supplier that sells seals.
Honda part 91201-402-015 if you want to go that route..
You will need a 16x1.5mm bolt to pull flywheel than remove starter clutch to get at crankshaft seal..
Left side engine cover should be totally oil free, weep hole designed into bottom of cover should not be plugged/sealed..