Cal. Gov. Code § 12058.5

In case of the death, disability or other failure to take office of the Governor-elect, whether occurring prior or subsequent to the returns of election, the Lieutenant Governor-elect shall act as Governor from the same time and in the same manner as provided for the Governor-elect and shall, in the case of death, be Governor for the full term or, in the case of disability or other failure to take office, shall act as Governor until the disability of the Governor-elect shall cease.