Motors are pretty simple machines though the causes for smoking are many.
Smoking on startup usually indicates either or both intake or exhaust valve seals.
If it smokes a little on start up and smokes worse when it gets hot, but it never fouls a spark plug and the plug has normal coloration, it's usually an exhaust valve seal.
If it smokes a little on start up and gets worse as it gets hot and it fouls plugs a lot, it is either intake valve seals or rings or both.
It might be the motor was not broken in properly and the rings are damaged. The most common mistakes during break-in is over heating and over revving. You can over heat the motor by letting it idle for more than 5 minutes. Over revving is self explanatory. During break-in it is OK to give it full throttle, just don't hold it at a high rpm, get off the throttle or shift. Riding down a road at high speed does both over heat and over rev. Riding too slow can over heat it too. Best break-in method is "pasture riding". Not too fast, not too slow, shifting up and down, make the engine pull and let the machine push the motor. Heat and cool cycles are important too. Ride for 15 minutes and let it cool for 30 minutes, ride for an hour, let cool for 2 hours.
In older engines I have seen oil rings worn out from dirty oil while the compression rings were fine because the air filter was serviced regurlarly.
Whatever the problem is, it can be corrected and remember - most problems are simple - humans just try to imagine the worst.
What kind of oil do you use? Synthetics are slicker than petroleum oils and get past the rings easier. Engines most generally should be broken in with petroleum oil. They will break in on synthetic, but it will take longer.