There's really only two things that could cause that to leak. Either your stator cover is cracked / warped and no longer has a good sealing surface on the left side of your case. Or you didn't remove enough of the old paper gasket off of the sealing surface when you replaced your stator causing it not to create a seal. (seen this 100 times) I am however assuming you did put a new gasket on when you reinstalled the stator cover, those are meant to be a one time use only.
2008 NINJA 250R TOTALED
2014 NINJA 300 SE SOLD
1992 NINJA GPX ZX600C PARTED OUT
1989 NINJA GPX 600R
2008 NINJA ZZR 600