The oil may be getting past the rings or past the valve guides if it is a four stroke motor. Rings and valves are in the top end of the motor, not the bottom. If the valve guides need replacing it should be done by a specialist machine shop.
Your rings are probably getting too worn. Instead of cleaning the oil off the side it is staying on and some is leaking by into the cylinders. Then it is being burned off during the combustion cycle. The only way to fix this is to tear the engine down and install a new set of piston rings.
As a bikes engine get mileage on it it is not unusual for oil to start leaking past the worn cylinder rings. Typically the only way to correct this is to take it apart, clean up the cylinders, and install new rings.
