My friends direct bike need HELP

My friend has a direct bike 50cc it was running great for months and it just went and now doesnt go over 20 mph changed clutch variator fuel and oil but still going 20mph any suggestions help

 

If it was running well and suddenly changed then I would start with cleaning the carb out, making sure jets are clean and throttle slide moves up and down fully, checking the variator rollers for flat spots, check the belt hasn't stretched, replace the spark plug.
If still no better then check the clutch springs are okay, check the clutch plates are not sticking, check the throttle cable is not sticking.
Follow that with checking the choke works and variator plates are moving fully in and out.