At 2030 eastern time, a Cessna 177RG lost engine power and made a forced landing in South Vienna. The pilot and passenger were seriously injured. The pilot said he was about seven miles from his destination of Madison County Airport at 2,500 feet when he noticed a burning smell and the engine lost power. He determined he could not make the airport and aimed at a dark patch on the ground. The airplane then impacted a soybean field in a nose-down attitude. Examination of the engine revealed the dual magneto had backed off its mounting bracket on the engine and was lying in the engine cowling. The attaching hardware for the magneto was not located.