I have been looking in to this and read 12t are OK but 13t some chains will need case grinding both will need longer chain. I have been thing about changing rear 42t to 35t when Treatland gets one in stock. That will use same chain but maybe loose some links. If you are using 10t x 42t going to10t x 35t is same ratio as 12t x 42t. From 4.2:1 to 3.5:1 Does your moped pull up a hill OK because you will loose some torque .