On June 6, 1998, at 1014 hours mountain standard time, an experimental Girgen Kitfox II aircraft, N62KF, experienced a loss of engine power while maneuvering. The pilot made a forced landing in an open dirt field near Peoria, Arizona. The aircraft, operated under the provisions of 14 CFR Part 91, sustained substantial damage. The private pilot/owner/builder and one passenger, the sole occupants, received minor injuries. Visual meteorological conditions existed for the personal flight and no flight plan was field. The flight originated from McGill Flight Park, Phoenix, Arizona, at 1000, and was scheduled to terminate at the Glendale, Arizona, airport. The pilot stated he and his passenger and another aircraft were traveling to Glendale airport for coffee. After takeoff, the two aircraft could not establish radio communications so they returned to McGill Flight Park to rectify the situation. Approximately 10 minutes later both aircraft took off with radio communications established. The pilot stated that after completing a 360-degree turn to wait for the other aircraft, the "engine came to a complete stop." The pilot stated that he was at 2,500 feet agl and he pushed the aircraft nose down to maintain airspeed and look for a place to land. He stated that he setup for an emergency landing in an open dirt field. The pilot said that he landed in a flat area about 300 yards long. After touchdown the landing gear was sheared off after colliding with a dirt berm. The aircraft traveled approximately 150 feet before coming to rest. The pilot exited the aircraft on his own, and helped his passenger out of the aircraft. The pilot reported that when the engine quit he had approximately 12 gallons of fuel onboard. An inspector from the Federal Aviation Administration (FAA) stated that the engine had been previously installed in a snowmobile. The snowmobile had been involved in an accident prior to installation of the engine in the accident aircraft. The owner inspected the engine following purchase and prior to installation in the airframe. No discrepancies were noted during the preinstallation inspection. Examination of the engine by the owner after the aircraft accident revealed that the rotary valve had broken under unknown circumstances. The engine manufacturer reported that the rotary valve is a circular plate made of heavy-duty aluminum located inside the engine. The plate has a hole matching the intake ports in each cylinder. As the plate rotates, the hole lines up with the intake port and allows the fuel/air mixture to enter the cylinder. According to the manufacturer, this engine had originally been installed on a 1989 Ski-Doo Mach 1 snowmobile, with a rated power of 100 horsepower. The engine manufacturer further stated that this particular model was not originally intended for use in an aircraft. Maintenance performed on the engine is a function of weekly, monthly, and yearly, or based on mileage (maintenance schedule appended to file). A general inspection should be conducted on a monthly basis, or every 500 miles.