Following the installation of an overhauled engine and the completion of an annual inspection, the pilot/mechanic that had performed the work elected to fly the airplane. During takeoff the engine indications were normal and the airplane climbed to about 500 feet above ground level when the engine experienced a sudden power loss and the rpms decreased. The pilot then maneuvered the airplane in a 180-degree turn back toward the airport. He overshot the runway and the airplane sustained substantial damage during an off-airport forced landing. Federal Aviation Administration inspectors who responded to the scene discovered that the throttle control linkage was not connected to the carburetor throttle plate arm; the associated bolt, castellated nut, and cotter pin were missing. The pilot/mechanic stated that he had connected the throttle control linkage but could not remember if he had installed the cotter pin.