On April 22, 2005, about 1645 Pacific daylight time, an Ercoupe 415-C, N2256H, registered to and operated by the pilot as a 14 CFR Part 91 personal flight, experienced a loss of engine power shortly after takeoff from George Felt Airport, Roseburg, Oregon. During the off airport landing to a grape vineyard, the aircraft was substantially damaged. Visual meteorological conditions prevailed for the local flight. Neither the private pilot-in-command nor his pilot-rated passenger were injured. The pilot-rated passenger (aircraft owner) reported that he did not hold a current medical certificate, therefore the private pilot in the right seat who was current was acting as pilot-in-command, however, he was at the controls at the time of the takeoff. The pilot/owner reported that the pre-flight check and start-up were normal. A magneto check with 1,800 rpm noted that the right magneto dropped about 75 rpm and the left magneto dropped about 100 rpm, which he reported as normal. At the time of takeoff, the aircraft attained an altitude of about 100 feet above ground level, when the engine began to gradually lose power, "like someone pulled the throttle or the throttle linkage broke." The pilot/owner initiated a forced landing to a grape vineyard. During the landing roll, the aircraft experienced a nose gear collapse, the firewall was damaged as well as the leading edge of the wings. On 4/26/05, an Inspection Authorization Airframe and Powerplant mechanic inspected the airframe and reported that the firewall was damaged as well as the nose gear fork was pulled out. The leading edge of the wings were dented to include damage to wing ribs and the fuel tank. The mechanic also inspected the engine and reported that a compression check was accomplished. The mechanic found that the 3# cylinder would only hold 35/80 psi, and there appeared to be an exhaust valve leak. The other cylinders were in the high 60's to 70's psi. The carburetor heat box was damaged from impact. The quick drain was cleaned off and fuel was present. Fuel was present in the fuel tanks and header tank. All spark plugs displayed normal operating signatures. On 4/27/05, the same mechanic prepared the engine for a test run. The engine was primed and started right up. The engine was warmed up for a few minutes at idle power and then run-up to about 1200 rpm. Oil pressure was good. The engine was started and stopped a few times, and then a magneto check was accomplished. The mechanic reported that the right magneto had a slight rpm drop, while the left magneto had a 300 rpm drop. The mechanic then shut the engine down and inspected the ignition harness and the top plugs. All looked ok. The mechanic removed both magnetos and associated ignition harness and sent them to the National Transportation Safety Board Northwest Regional Office, Seattle, Washington. On 5/6/05, both Magnetos were hand carried by the NTSB IIC to Galvin Flying Service Maintenance Accessory Shop, Seattle, Washington. Both Eisemann Model LA-4 magnetos and associated ignition harnesses were visually inspected and prepared for a test-stand functional test. Both magnetos produced strong spark from all towers up to 3,000 rpm with no mis-fire. The harnesses were then tested using a high tension lead tester with no defects noted.