According to the pilot, the airplane had returned to service after being restored by an airframe and powerplant mechanic with inspection authorization. The pilot performed several takeoffs and landings prior to the accident flight, and noted that the airplane seemed to operate normally. The pilot departed to another airport where he completed a simulated instrument approach to about 50 feet above the runway. He then commenced a go-around over the departure end of the runway with the airplane about 300 feet above the ground. The engine experienced a partial loss of power and the pilot maneuvered the airplane to a hay field, where he performed a forced landing. During the landing roll, the horizontal stabilizer struck a hay bale resulting in substantial damage to the empennage. A postaccident 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, which resulted in excessive play in the throttle linkage.