A bigger front sprocket and smaller rear sprocket will get you going faster at top speed but it will take you longer to get there, also an acceleration or pumper carby will help too. Smaller engines are more subtle in their capabilities compared to stock as 25% faster than 100kph is 125 kph, so polishing the intake port will improve it slightly as will a more powerful spark and bigger carby and better exhaust flow, after the mechanical things are taken care of you can move onto aerodynamics to improve speed along with suspension to improve speed through cornering, and then tyres can not be left out and should not be left until last.