How fast does a 49cc 4 Stroke engine go?

well i have my 140cc dirt bike set up with a 49cc 4 stroke ATM, and its putting i think 50 kmp. id say with the right gear ratio you might be able to get something going 70kmp, my gearing is set for 140cc, (im only hitting low end of 3rd gear- it dosent have enough torque) but yea with the right gear set up, prob 70! GL