i will suggest you to first take advise from a person who knows about bigger bikes engine like 250cc 400cc 700cc etc as far as i know every 250cc have different frame size etc so you must take multiple 250cc engines ager 150cc k frame me ajata hai tu gr8
i used to have chineese bike in heavy bike look u can see in my rides ( garage ) ik he yamaha ki engine us ki frame me aarha tha woh yamaha ka tha per 2stroke is ki wajah se alteration se me bhag rha tha k na ho aur chineese bike thi so kafi log dara b rhy thy
mujhe itni knowledge hai nhi per jo upper likha hai yahan k he kuch members ki advise thi
best of luck