You can't bore the cylinder oversize because they are a plated cylinder. You can re-plate the cylinder, or have a sleeve installed. If the guy told you it was over sized to a 100cc, then he most likely had a big bore kit installed. You or the shop will have to measure the piston and the cylinder bore to find the correct size, then locate an aftermarket piston that fits the cylinder properly. Wiseco, JE, Pro-x are good pistons. They might make a piston to fit your cylinder. Try to find out what brand of big bore kit was used. You can get a replacement piston through that manufacturer as well.