the variator is probably stuck as the rollers wear and either fall out in there and jam it up or wear into the side of the ramps.does it go ok on a straight?
Either the variator is stuck in the geared up position usually due to the rollers being worn or the belt is too short.is the throttle cable ok and not jammed with some free play in it ?
This is the manual for the keeway Outlook 150 cc which is basically the same as the 125 cc.
https://www.scribd.com/doc/238408121/Manual-de-Taller-Keeway-Logik-Idioma-Ingles