CA DMV requires the information be sent to them if you swap a motor in your car.
Engine or Motor Change: Distinguishing Vehicle Identification Number
4161. (a) Whenever a motor vehicle engine or motor is installed, except temporarily, in a motor vehicle which is identified on the ownership and registration certificates by motor or engine number or by both the motor and frame numbers and subject to registration under this code, the owner of the motor vehicle shall, within 10 days thereafter, give notice to the department upon a form furnished by it containing a description of the motor vehicle engine or motor installed, including any identifying number thereon and the date of the installation. The owner of the motor vehicle shall also submit to the department with the notice the certificate of ownership and registration card covering the motor vehicle in which the motor vehicle engine or motor is installed and evidence of ownership covering the new or used motor vehicle engine or motor installed and such other documents as may be required by the department.
It's a pain in the ass to do anything motor-related in CA.