Your not too clear on what's lagging. If, when you punch the throttle, does it hesitate, or does the engine immediately rev up, then the clutch catches up with it ? If it's the latter, you've probably got worn out clutch plates or oil on them, in any case it'll have to be taken apart and inspected for wear. You might get some more use out of it by taking the glaze off the clutch plates with fine sandpaper. If it's similar to a Peugeot clutch, and I'm sure it is, you'll want to make sure the steel balls in it aren't corroded and capable of moving freely. They should be lightly coated with a high temperature grease to prevent rust and promote free movement. If the engines hesitating, naturally the clutch will be slow to engage, you'll have to look into why the engines not picking up speed quick enough. I would think a new plug and decarbonization would be a good place to start. Don't worry about smoke from the exhaust, all 2 strokes do it.