Are you used to riding 2 strokes, they need a bit of throttle to get the engine pulling well.
Have you tested for air leaks, low power can be a symptom with power loss at take off?
Are you getting the engine to operating temperature before you attempt to ride?
Are you allowing the clutch to slip and then take up, or are you just letting it out quickly.
Does the oil in the gearbox seem thick and sticky.