There are alot of issues you will come accross. There are some bikes you can easily upgrade such as a ktm duke 125cc and engine swap to the 390cc and also the yamaha mt125 to the mt03 engine as the frames are the same but you will also need to upgrade the brakes to the specification of the upgraded engine to accomodate the extra speed. Some older bikes are alot easier to convert sometimes may require adjustments to the frame mounts and will most likely require welding. The ajs daytona regal raptor 125s can be converted a few engines mostly 250cc and iv even seen 350cc and i think there honda engines. These are the older models with carb i think it would be tricky with the new fuel injected highway star. Another 125cc that can be easily modified is the hyosung GT 125 and the frame will take the 250 and 650cc engine. The simple answer is you can drop a bigger engine into most bikes its just a case of making the nesersary modifications. And thats not just a bigger engine. You have to account for breaks, alterations to the carb, potentially a new exhaust and more. Also a longer swing arm. Upgraded forks the list goes. Id only reccomend it as a project as it wont be cheap!