could be one of 3 things,
1, the air filter is blocked of the airflow to the carb is restricted for some reason
2, the exhaust has a collapsed baffle or an interal part is basically blocking the flow of gases for the engine
3,the rocker arm has broken for one of the valves (if it has more than 2 valves only)it will still run with only 1 inlet valve for instance opening but be slow.i have seen it before on honda and aprilia scooters.
adam bike doctor
January 2010