12 1/2 x 2.75 Inner Tube fits majority of mini dirt bikes. Be sure that the valve stem is poking out of the rim straight, before inflating the inner tube. If you inflate the tube with the valve stem crooked, the tube will get damaged and burst.
Often, the pressure ratings on the sidewall of Chinese mini dirt bike tires are incorrect. Do not even think about putting anywhere close to 60 psi into the tires! Only inflate as much as needed depending on the rider weight. This will usually be 25-35 psi.