I assume your asking about a 12mm crank that your kit did not include. I bought a bunch of parts from a vender in china and everyone wanted the 10mm cranks when i was selling off my stuff. Either get a piston with a 10mm wrist pin or a 12mm crank. Or get a better big bore kit!
What carb came with that kit?
Send it back if you can that kit is dodgy notice how it came with a windowed piston but the cylinder is not ported for it no crank and the del rep carb that is worthless and falls off due to engine vibration. The only difference between the red coil and the stock coil is the outside is red.
The question should be - how do i fix this turd and/or should i buy a 10mm piston.
You should of bought a better reed - exhaust and all the other go fast part before tearing down the motor. When you did tear down the motor, you should of had someone machine a port into the head. You can slap your motor together, but its not going to make as much hp as a built motor. A built motor with a 40mm oem one piece could eat what your going to build in your garage