On February 17, 1995, at 1235 central standard time, a Cessna 172RG, N5096U, was substantially damaged during a forced landing near Plano, Texas. The commercial pilot and his passenger were not injured. Visual meteorological conditions prevailed for the repositioning flight from McKinney, Texas, to Addison, Texas. The airplane was in a descent 7 miles northeast of Addison Airport when the engine lost power. After attempting unsuccessfully to restart the engine, the pilot landed in a plowed field. According to the pilot, when the nose wheel touched down in the soft dirt it "dug in and broke off" and the "airplane performed a forward somersault and came to rest inverted." A Federal Aviation Administration inspector reported that the right wing spar was bent and the upper portions of the vertical stabilizer and rudder were crushed. Examination and disassembly of the engine on 03/06/95 revealed a lack of gear train continuity. The crankshaft gear lockplate tang was sheared and the crankshaft gear retaining bolt had backed out of the crankshaft to the point where no threads were engaged. The crankshaft gear alignment dowel was fractured. An examination of the aircraft logbooks by a Federal Aviation Administration inspector revealed that the propeller had been replaced on 02/13/92 following a nose gear collapse. Airworthiness Directive (AD) 91-14-22 was not complied with at that time. The AD requires an inspection of the crankshaft gear following any propeller strike which results in sudden engine stoppage. According to the operator, the nose gear collapse did not result in sudden engine stoppage.