On August 9, 1996, approximately 1600 Pacific daylight time, a Cessna 172N, N738PM, sustained substantial damage when it flipped over during an emergency landing near Enumclaw, WA. The commercial pilot was uninjured. The pilot rated passenger received minor injuries. The flight was conducted under 14 CFR 91 in visual meteorological conditions. No flight plan was filed for the flight which had originated at Boeing Field, Seattle, Washington, approximately 1345 hours. The flight was an aerial observation flight. The pilot reported that he was in a 300 ft/minute descent when the engine made a strange noise and began to vibrate out of the ordinary. The RPM dropped to 1900 and stayed there for about 20-30 seconds. The power then dropped to 1500 RPM. The pilot established a glide speed and turned toward the Howard Hanson Reseviour. When the RPM dropped to 1000, the pilot made a forced landing. During the landing in grass and uneven terrain, the airplane flipped over. Inspection of the engine by FAA inspectors revealed that the cam follower in the magneto drive had failed due to a lack of lubrication.