On June 20, 1997, about 1840 central daylight time, a Schweizer G164B, N6816Q, registered to Grays Flying Service, Inc., collided with trees while making a forced landing following loss of engine power at Edwards, Mississippi, while on a Title 14 CFR Part 137 aerial application flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft received substantial damage and the commercial-rated pilot was not injured. The flight originated from Edwards, the same day, about 1830. The pilot stated during pull-up after an application run, the engine lost power. He made a forced landing in a soybean field and could not stop before colliding with trees at the end of the field. Postcrash examination of the engine by an FAA inspector showed the right magneto had malfunctioned. The magneto was replaced and the engine was operated to full power with no further evidence of failure or malfunction. Examination of the right magneto showed the distributor drive gear had several gear teeth missing. The gear would not turn the distributor on occasion, causing the magneto to lose timing with the engine.