On May 19, 1993, about 0955 central daylight time, a Piper PA-25-235 airplane, N8787L, registered to Northern Aviation, sustained substantial damage following a loss of engine power and a forced landing in a field near Mexico, Missouri. The solo commercial pilot received serious injuries. The 14 CFR Part 137 aerial application flight departed Mexico, Missouri, about 0950 in visual meteorological conditions. The pilot reported a partial loss of engine power during a spray run. He said the engine coughed, and he saw black smoke come from the exhaust. He said he applied carburetor heat briefly and when that did not restore full power, he first pumped the throttle and then leaned the mixture. The engine continued to lose power, and a forced landing was made in a soft, fallow field. The airplane landed hard and nosed over. A witness to the accident recalled hearing the engine misfire just prior to the forced landing. The airplane was refueled at the departure airport shortly before the accident flight. On June 15, FAA inspectors observed an engine run of N8787L's engine. The engine had been removed from N8787L and installed on another Piper PA-25 airframe. During the engine removal, the mechanic doing the removal observed water in the fuel gascolator bowl. He also said he had to change the carburetor air box and remove the left magneto for the engine to fit on the new airframe. When the left magneto was removed, the mechanic noted oil had leaked into the casing, which he attributed to the position of the engine during shipping to his shop. The mechanic also replaced a broken high tension lead on the right magneto, which he thinks was broken during engine removal or shipping. The FAA inspectors observed the engine run at idle for approximately ten to fifteen seconds, and then increase to full rated power of 2650 RPM with slight roughness. No mechanical anomalies associated with the engine were discovered during the engine run.