sounds like you have several prblems, the engine idling fast and needing choke a lot and the oil coming from carb joint may all be related, you need to carefully investigate this situation, relace any joint gaskets in that connection but before you do just ensure the screws hoding this joint are tight enough
you also need to remove the exhaust, replace the joint there and align the thing better so there are no gaps, this may also give to poor running
as for oil in the airbox, clean it all out and check there isnt too much oil in the engine, there maybe an inspection window but bike has to be upright not on sidestand