How many miles have you put on it now with the white smoke??
The least expensive problem would be..... If the air filter is over oiled, first the oil will fill up the drain tube under the airbox. Then it will dribble down into the carb and cause smoke. I overdid it with k&n filter oil. Two years later I still have red oil dribbling into my airbox drain.
Is the spark plug carbon foiled from running rich or oil fouled from a bad seal somewhere?
You Might Try a leak down test.
The test is simple. Apply air pressure to the spark plug hole and listen. (make sure the valves are closed) You might have to take the exhaust pipe and carburetor off plus remove the oil fill cap. Other then that, just listen. You will need at least fifteen or twenty pounds of air pressure. Higher pressure will make the sound louder but too much and you will need to lock the crankshaft to keep the engine from turning. A hissing noise at the carburetor manifold indicated a leak at the intake valve(s). Hissing at the exhaust, leaking exhaust valve(s). Hissing at the oil fill or at the engine breather tube, leaking past the rings.
http://www.motorcycl...own-Tester.aspx A handyman can make one of these for about 20 bucks