i too have a 125cc baja dirt runner and i want to make it faster aswell but the thing is nothing will fit to it so it's kinda hard. Someone said to get a vermon motors exhaust on it but he said it might not fit.
I did this mod on my panterra 125 - i don't know if this is the same as the dirt runner, but was able to get the speeds he is talking about. I want to change my 43 tooth out to a larger size, so I'm able to sell mine. PM if interested.
what i did was take off the air filter box clamp on a k&n air filter replaced the spark plug cable with a nology hotwires one and gutted the exhusath oh and the suppertrapp fiter does fit a budy of mine has it on his 125 but put a ttr 125 tip on mine