My KMX125 just cooked.. can someone advise me with what I will need to do fix it?

if you did not do to much damage to the cylinder you can get the cylinder honed out and install a new piston and rings. if the cylinder is damaged bad enough then it will need to be bored out to the first oversize piston which is 10 thousandths over stock. hopefully you did not any damage to the bottom end. putting in a new piston is not that hard to do if you have some Mechanical knowledge if you do not then let your local motorcycle dealer fix it.