On September 4, 2003, at 1530 central daylight time, a Piper J3C-65, N38435, contacted soft terrain during an off-airport forced landing following a loss of engine power near South Shore, South Dakota. The pilot and passenger were not injured. The airplane was substantially damaged. The 14 CFR Part 91 personal flight was operating in visual meteorological conditions without a flight plan. The flight originated from a private airstrip in Summit, South Dakota, at 1500, with an intended destination of Milbank, South Dakota. The pilot reported he had filled the aircraft with fuel at the private airstrip just prior to takeoff. Approximately 15 minutes after departure the engine lost power. The pilot stated he pumped the throttle, checked the fuel valve and magnetos but was unable to get the engine restarted. The pilot stated the airplane contacted soft terrain during the forced landing. The airplane was fueled using above ground fuel tanks at the private airstrip, which is also used by an aerial application operator. There were no reports of problems with other airplanes that had been refueled using the same source. A mechanic who inspected the airplane after the accident reported the wings and landing gear had been removed. He stated the header tank was empty as was the fuel strainer. He was unable to identify any failure/malfunction which would have resulted in the loss of engine power. The mechanic added fuel to the header tank. The engine was started and run up to 1,500 rpm prior to the mechanic shutting it down.