Hucky95

Hello, I have a sherco ST125 2011 and i am wanting more power, i was wondering what the difference is between the 125 and the 250 to see if i can just swap engines and possibly another tooth on the drive gear? the only difference i can see is the head, piston and con rod? am i correct in thinking this or is the carb different too? i have seen a post saying the carb was slightly different but only on the main jet. hopefully someone can shed some light on the situation. Cheers