According to the pilot, the airplane had returned to service after being restored by a licensed airframe and powerplant mechanic with inspection authorization. The pilot performed several takeoffs and landings prior to the accident flight, and the airplane seemed normal. The pilot subsequently took off again and flew to another airport where he completed a simulated instrument approach to about 50 feet above the runway. He then commenced a go around and was over the departure end of the runway, about 300 feet above the ground, when the engine had a partial power loss. The pilot maneuvered the airplane to a forced landing in a hay field, and during the landing roll, the horizontal stabilizer struck a hay bale resulting in substantial damage to the empennage. A subsequent examination of the wreckage by a Federal Aviation Administration inspector revealed that a cotter pin was not installed on the throttle arm retaining nut. The throttle arm retaining nut was finger tight and the throttle arm was loose.