At about 1445 Pacific time, the aircraft sustained substantial damage following an in-flight loss of engine power and subsequent off-airport, forced landing. The Private pilot, the sole occupant of the airplane, was not injured. The pilot reported that approximately 15 minutes after takeoff, while in straight and level flight at 4000 feet msl, the engine began to run rough and eventually quit. After experiencing the loss of power, the pilot initiated a landing to a nearby open field. During the landing rollout, the airplane encountered tall grass and nosed over, resulting in substantial damage.