California statues on License plates:
License Plates
4850. (a) The department, upon registering a motorcycle, shall issue to the owner one partially or fully reflectorized license plate.
4852. Motorcycle license plates shall measure seven inches in length and four inches in width, and the characters on the plates shall have a minimum height of one and one-half inches and a minimum width of nine-sixteenths inches, and shall have a minimum spacing between characters of three-sixteenths of an inch.
5004.5. Any owner of a motorcycle manufactured in the year 1942 or prior thereto shall be issued special license plates for the motorcycle.
5060. (2) Special interest license plates authorized under this article may be issued for use on a motorcycle. That license plate shall contain a five digit configuration issued in sequential numerical order or, pursuant to Section 5103, in a combination of numbers or letters. There shall be a space to the left of the numerical series for a distinctive design or decal and the characters shall contrast sharply with the uniform background color. No motorcycle plate containing a full plate graphic design is authorized. Those particular special interest license plates that were issued prior to the discontinuation provided by this paragraph may continue to be used and attached to the vehicle for which they were issued and may be renewed, retained, or transferred pursuant to this code.
5061. (a) Notwithstanding any other provision of law, if the department permits the issuance of a special interest license plate for display on a motorcycle, the department shall not approve any design for that plate that incorporates either or both of the following:
(b) Any special interest license plate issued for display on a motorcycle is subject to the same fees that are collected for the issuance and retention of special interest license plates on other vehicles.
5101.3. (a) Any Pearl Harbor survivor may apply for special license plates which shall run in a separate numerical series and shall contain the words "Pearl Harbor Survivor." The plates may be issued for any vehicle, except a vehicle used for transportation for hire, compensation, or profit, or a motorcycle.
An overall summary is here:
http://www.ducatigirl.com/dmv/dmv_a_main.html