there are a few things it could be ill put them in order of most probable first,
The diaphragm in the top of the carb could be split and not lifting the slide up with air when you open the throttle ,it can also not be seated properly around the top of the carb and diaphragm cap ,this will also cause an air leak.
The slide could be stuck in the carb.
The air filter could be badly blocked.
The exhaust could have collapsed inside and blocked up and stopping the gases from escaping.
The valve timing may have slipped ,its unlikely but possible if the cam chain is loose.
The cdi unit is limiting the revs,this can happen if the rev limiter in the cdi is cutting in too early due to being faulty,again not likey but possible.
adam bike doctor (milton keynes motorcycle centre)