Im a bit confused
You said you had it pinned, backed off and it WAS SMOKEY ? This to me does not indicate a failure. That should be a good sign. Bad if its a 4-stroke, excellent for a 2-stroke. Means there is plenty of oil in there.
2-stroke kart engines have alot of seizures. They are probally the most stressed 2-stroke engine out the box. Some of the faster motor rev to 18000 plus RPM !! Common to see at the end of the back straight, off the gas seizures, because when you shut the throtle, you loose nearly all fuel (and oil) in the engine.
Dirt bike motors also get thrashed. Image the shock on the motor when landing a jump on full throtle, and it pulls the motor down from 10000 to 6000 in a fraction of a second. Acelleration and decelleration of the piston from one extreme to the other would shorten engine life too. A street bike generally would not see that sort of thing.
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