150 cc GY6 won't go up after 6000 rmps

[kochun17]

I have a KINROAD Scooter - XT50QT-10. After couple months for leaving, my scooter was damaged by some A holes. Which a vacuum line, oil line and a wire were cut. I replaced all lines, and clean the carburetor thoroughly, and placed a new spark plug. It runs again, but it won't go more than 6000 rmps. It lost power if I throttle it after 6000 rmps. I have done air fuel mixture ratio and air enriched ratio adjusting. It goes a little more, but still not the same scooter as I had before. Anyone has any idea, please ?