On September 22, 1995, about 1030 eastern daylight time, a Grumman G164A, N9700, nosed over during a forced landing near Lynchburg, South Carolina. The airplane was operated by Sky Ag Aviation, under the provisions of 14 CFR Part 91, and visual flight rules. Visual meteorological conditions prevailed. A flight plan was not filed for the ferry flight. There were no injuries to the commercial pilot, and the airplane was substantially damaged. Origination of the flight was at approximately 1000, from Lynchburg, South Carolina. The pilot stated that during the ferry flight from spraying a field, the engine began running roughly. He checked the magnetos with no improvement in the roughness. The pilot then added power, and the roughness increased, resulting in a forced landing in a soybean field. During the landing roll the airplane nosed over. Post accident investigation displayed that the engine would not start with the magnetos turned to the both position. With the magneto switch placed in the left position, the engine started and ran steadily. Further investigation disclosed that the left rear attachment bolt for the right magneto was missing, and the safety wire was broken. The right rear corner of the magneto was broken, allowing the magneto to shift enough to jump the teeth of the magneto to the engine drive coupling. This resulted in a loss of engine to magneto timing. The fracture faces of the broken magneto mount were dirty and fretted, except for one small area: indicative of a pre-existing fracture.