On July 16, 2014, about 1450 eastern daylight time, a Cessna 172N airplane, N13SK, was substantially damaged during a forced landing near Rockville, Indiana. The pilot and passenger were not injured. The airplane was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Day visual meteorological conditions prevailed for the flight, which operated without a flight plan. The local flight departed from a private airport near Rockville, Indiana about 1400. According to the pilot, the engine lost partial power while in cruise flight. The pilot attempted to restore engine power by applying carburetor heat and checking the mixture and fuel selector positions, but engine performance continued to worsen. The pilot performed a forced landing into a corn field, during which both wings were damaged. During examination of the engine, both points of the single drive/dual magneto system would not open; as the center cam was rotated to the 'points open' position, both points remained closed. Further examination revealed that four screws designed to hold the magneto points in their operational position had become loose. Two of the loose screws did not have any torque paint on them; torque paint for the other two loose screws was missing from the screw slot. Aircraft records indicted the airplane had flown eight hours since its last annual inspection on February 3, 2014. The investigation was not able to interview the mechanic who performed this annual inspection and recent maintenance on the airplane.