On August 30, 2016, about 0855 central daylight time, a Republic RC3 amphibian airplane, N6490K, conducted a forced landing near Post Oak, Missouri. The private rated pilot received serious injuries and the airplane was substantially damaged during the accident. The airplane was registered to and operated by a private individual, under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed at the time. The pilot reported to the Federal Aviation Administration (FAA) inspector that the flight had just departed from a private airfield. During the flight, he felt the engine had lost power in a cylinder and tried to return to the airfield; however, he could not maintain altitude and elected to conduct a forced landing in a field. During the approach to the field, the airplane lost altitude and impacted trees, coming to rest nose low among several trees. An on-site examination of the airplane revealed substantial damage to the airplane's fuselage and wings, and fuel was present on site. An examination of the airplane's engine was conducted by the FAA inspector. The inspector noted that during a cylinder compression check of the engine, the No. 1, cylinder exhaust valve was not seating, and air was leaking past the piston rings on the other cylinders. He noted that the compressions on all the engine cylinders were low. He added that the cylinders appeared to have rust in them. He also reported that the pilot said the airplane sat for about 4 years before the airplane was returned service. The airplane had flown about two hours after being returned to service, before the accident flight.