On May 25, 2019, about 1430 central daylight time, a Rockwell S2R-10 airplane, N4907X, sustained substantial damage when it was involved in an accident near Oakes, North Dakota. The commercial pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The pilot reported that he was spraying a field about 10 miles southeast of his home airport. After completing 15 spray runs, the pilot maneuvered the airplane for another spray run when the engine lost total power and the pilot could not maintain altitude. The pilot maneuvered the airplane for a forced landing in a field, during which the fuselage and wings were substantially damaged. The pilot exited the airplane without injuries. The engine had about 485 hours of time in service since its most recent overhaul, and about 12 hours since its most recent annual inspection. The engine was successfully operated in a test cell at idle power. After the engine run, the front section of the engine was disassembled and examined. The helical cam gear was found with its gear teeth fractured and smeared. Several of the gear teeth cracks showed fatigue signatures during metallurgical examination. The helical cam gear assembly did not show any pre-existing installation anomalies and no other pre-impact anomalies were found with the engine.