i have a lifan 110cc engine in my pitty an have hit 80-85 km/h but it depends on what type of gearing you have on it cause i ride on alot of straights so i changed my gearing so i get more top speed, i raced a guy on a atomix 125cc and i bet him on the straight but around a track he would beet me so yeah you can get all types of speed out of ya 125 if the gearing is standard id say you should hit 80-90 km/h
oh an try to get a lifan 125cc engine not a ducar 125 cause the lifans are more durable