I'm currently working on a 2013 YZ450F with a similar issue. A friend said he was having trouble starting his bike, specifically the kick starter seemed to be locking up. In the process of inspecting the Bike I found large chunks of metal in the oil, see attached picture. After splitting the casings, I've been unable to identify where the metal fragments have come from, although it appears as though someone was filling an oil access port between the casings and I am now wonder if the factory left FOD (Foreign Object Debris) or now Foreign Object Damage within the engine. I'm still searching for the culprit of the metal fragments and the cause of the kick starter locking up, that's another post for me. Sorry if this isn't any help, once I find the issue I'll update my post.