First, verify your
air-filter is not soaking with oil. Only need two-layers of spray-oil per side.
Next, with bike level and rested overnight, make sure it's not over-filled with oil. You should see bubble of top of oil-level in sight glass.
Make sure crankcase vent-tube is NOT clogged. That's tube connecting rear of engine to air-box at bottom.
Do warm compression-test first and post results here. Do dry and wet test with oil squirted down spark-plug hole. Depending upon tester used, it should read anywhere from 120-180psi.
Then do leak-down test to determine if it's rings or valves that's leaking. You might just need valve-adjustment.
Last ditch-effort, do piston-soak and see if it improves.
https://www.ninjette.org/forums/showthread.php?t=343925