On August 23, 2004, at 0950 central daylight time, an Enstrom F-28A helicopter, N9573, sustained substantial damage when it impacted terrain and rolled over after a loss of engine power during takeoff near Leroy, Minnesota. The pilot received serious injuries, but the passenger was not injured. The 14 CFR Part 91 aerial observation flight was departing to conduct a local observation flight. Visual meteorological conditions prevailed. No flight plan was filed. The pilot reported that the engine was producing 3,000 rpm's when he brought the helicopter into a hover. He reported that he transitioned into forward flight and cleared a fence, and then the engine lost power. He reported that he thought that an exhaust valve was stuck. He attempted to do a run on landing to a field. The helicopter touched down on its left skid two times but the helicopter was not straight. He reported that the third time the helicopter touched down on its left skid, the helicopter was not straight and the rpm's were low. He reported that he ran out of tail rotor authority. The helicopter rolled over on it left side. A Federal Aviation Administration airworthiness inspector examined the engine at the Marion Airport, Marion, Iowa, on September 1, 2004. Representatives from Enstrom Helicopter, Textron Lycoming, and the operator attended the engine inspection. An engine run was conducted. The engine was run at 1,800 to 2,000 rpm, and then at 1,110 to 1,200 rpm; but the engine popped and ran unevenly. The engine inspection revealed that the 1/8-27 NPT Allen plug to the number four cylinder was missing. Once the hole was plugged, the engine ran smoothly.