Lean mixture does tend to cause 2-strokes to lose power without running roughly, but usually you would notice that it revs higher if the mixture is lean.
If you suspect that it does tend to rev higher as well as lacking power, the three O rings are a possible (and fairly likely) cause. One O ring goes at the carburetor end of the intake pipe, one at the cylinder end of the intake pipe, and the big one goes under the starter and seals the side of the crankcase. Any one of them could cause lean mixture (and all three of them often do so). However the fuel tank, fuel line and carburetor are also likely causes of lean mixture.
One important question is, what did you change between when it worked properly, and now?
