The only draw back in my mind for the 170cc kit on the 125cc engine would be in the head. If the 170 kit has a head/piston it may be a good set up. If it is still relying on the 125cc head I personally would not use that kit. The whole set up will be around 700-1000 bucks by the time you install a carb/intake, exhaust,air filter. It would be a lot more powerful, and bottom line is that is what we all want!