On July 26, 2003, about 0650 eastern daylight time, an Air Tractor AT-301, N23820, was substantially damaged during a forced landing, after a loss of engine power near Rhinebeck, New York. The certificated commercial pilot sustained minor injuries. Visual meteorological conditions prevailed and no flight plan had been filed for the flight that departed the Green Acres Airport (1A1), Livingston, New York. The aerial application flight was conducted under 14 CFR Part 137. The pilot reported that the airplane was in cruise flight at 1,200 feet msl, when the engine began to vibrate and lose power. The pilot was not able to maintain altitude, and performed a forced landing to a field. During the landing, the engine and right main landing gear separated from the airframe. In addition, the left wing was damaged. The airplane was equipped with a Pratt and Whitney R1340 radial engine. Examination of the engine by a Federal Aviation Administration inspector revealed that approximately 50 percent of the number eight cylinder head was separated from it's respective cylinder barrel. Review of maintenance records revealed that the engine had been operated for about 930 hours since it was overhauled on May 20, 1998. At the time of the overhaul, nine new cylinders were installed. The airplane had been operated for about 20 hours since it's most recent annual inspection, which was performed on April 8, 2003. At the time of the inspection, a mechanic reported that he examined the cylinders and did not detect any cracks.