the only stuff that exists for a moped in california are:
the moped license plate
and
the "moped identification card" which serves as the title/bill of sale/pink slip. it's just a white piece of paper with blue and white text on it. you are supposed to carry a copy of this while riding. you are also supposed to fill out the transfer information at the bottom when you sell it to someone else.
you do not need anything else.