While conducting an aerial application swath run the engine lost all power and the pilot conducted a forced landing in a field with high vegetation which was unsuitable for a forced landing. Examination of the engine provided evidence of failure of both the left magneto drive gear and crankshaft idler gear. Examination of the aircraft records provided information that over 247 hours had been accumulated on the aircraft since the last 100 hour inspection and that an Airworthiness Directive directed at the magnetos had been complied with.