On December 23, 1994, at 1400 eastern standard time, a Cessna 150G, N3221J, collided with a ditch during a forced landing at Rhine, Georgia. The commercial pilot and his passenger were not injured. The aircraft was substantially damaged. The aircraft was operated under the provisions of 14 CFR Part 91 by the pilot. Visual meteorological conditions existed at the time, and no flight plan was filed for the personal flight to Columbia, South Carolina. The flight originated in Tallahassee, Florida, at 1230. The pilot reported that 90 minutes into the flight, the engine began to vibrate, and the engine rpm dropped to 2,100. He checked the magnetos, carburetor heat, and mixture; everything appeared stable. About three minutes later, the rpm dropped to 1,700, and he prepared for an emergency landing. He landed the airplane on a small agricultural landing strip. He was unable to stop the aircraft in the remaining runway, and the aircraft collided with a ditch. After the accident, the engine was examined by an inspector from the Federal Aviation Administration. He reported that both rocker shaft bosses of the number two cylinder had failed, and were broken away from the cylinder head. He observed no evidence of fatigue on the fracture surfaces. An examination of the maintenance records revealed that Airworthiness Directive 94-05-05, which requires inspection of the rocker shaft bosses, had been complied with. The reason for the rocker shaft boss failures was not determined.