On May 10, 2002, about 1850 central daylight time, a Rockwell S-2R, N4224X, piloted by a commercial pilot, was substantially damaged during a nose over during a forced landing after a loss of engine power near Bancroft, Wisconsin. The 14 CFR Part 137 aerial application flight was operating in visual meteorological conditions and was not on a flight plan. The pilot was not injured. The local flight originated at 1820. The pilot reported that he, "...filled the left [fuel] tank and sumped tanks immediately." The pilot was spraying at approximately 10 feet agl when the engine, "...became rough, backfired and quit." The pilot reported that he turned on the fuel boost pump and turned off the spray pump while initiating a climbing turn toward the airport. The pilot reported that the engine started producing power again. At approximately 150 feet agl the engine suffered another loss of power. The pilot stated he prepared for a forced landing. The airplane's tailwheel contacted the ground first followed by the airplane's main landing gear. The airplane then nosed over and slid to a stop inverted. Examination of the fuel system after the flight found sediment contamination in the main fuselage fuel tank. The carburetor fuel filter was also obstructed by sediment. The source of the contamination was not found. No other anomalies were found with respect to the airframe, engine or systems that could be identified as existing prior to impact.