Yup its possible u needs to do following steps,
1) Get Carank case opening position grinded to accomodate the slightly bigger Cylinder/sleeve.
2) Get rear outer surface of GTO 125 cylinder (Sleeve) grinded to reduce the circumfirence.
3) Get GTO 125s piston fitted at con rod.
4) For more increase in power bigger jets from GTO 125 Carb can also be used.
These four steps are not quite simple but can be done by any specilaist mechanic of Kawasakis