On March 14, 1998, at 0910 central standard time, a Grumman-Schweizer G-164A, N8626H, nosed over, according to the pilot, during a full stop landing on a county road near Marianna Florida. The aerial application flight operated under the provisions of 14 CFR Part 137 with no flight plan filed. The pilot reported that visual weather conditions prevailed at the time of the accident. The commercial rated pilot received minor injuries. According to the pilot, the airplane sustained substantial structural damage. The flight departed the staging area near Marianna, Florida, at 0905. Approximately five minutes into the flight the pilot reported that the engine developed a rough running condition. The pilot tried several power settings in an effort to reduce the condition, but no action on his part resolved the rough running condition. Since the pilot could not maintain altitude, he then selected an adjacent field for an emergency landing. The pilot said that landing surface was rough and uneven. Examination of the engine assembly disclosed that the right magneto cover was not secure and that the five screws that secure the cover to the magneto assembly were missing. The visual examination of the internal components of the magneto assembly showed evidence of water stains on the magneto block. Internal component continuity was established when the engine was rotated completely through. Low compression was also noted on all cylinders. A small quantity of fuel was recovered from the carburetor. According to the aircraft wreckage recovery crew, a substantial amount of fuel was recovered from the wing tanks at the accident site.