On November 11, 1998 about 1700 eastern standard time, a Grumman-Schweizer G164B, N6656Q collided with tree stumps and the ground during a forced landing near Cuthbert, Georgia. The aerial application flight was operated by the pilot under Title 14 CFR Part 137 and visual flight rules. Visual meteorological conditions prevailed at the time of the accident and no flight plan was filed for this flight. The commercial pilot received minor injuries, and the airplane was destroyed. The flight originated in Cuthbert, Georgia, at 1655. According to the pilot, he was flying 500 feet above the ground and was repositioning the airplane to conduct a spraying operation when the engine lost power. The pilot selected a forced landing area and executed a forced landing. During the emergency land, the airplane collided with tree stumps and nosed over. During the engine examination there was a fatigue fracture noticed within the fuel control gear shaft. A crack was detected in the web portion of the gear shaft, and heavy wear was noted on the loaded side of the outer dimension (OD) gear teeth and on the shoulder on the OD surface of the shaft brackets. During the fracture examination, it was noted that the "peeling" type fatigue fracture, and the angling of the final separation through the shaft were typical of torsional loading. The failed gear shaft provided direct drive to the engine fuel pump. During the wreckage examination by FAA Inspectors, it was discovered that the fuel control and the fuselage fuel filters were clogged. An undetermined amount of Jet fuel was discovered at the accident site. Fuel was also found throughout the airframe fuel system. A review of the airframe maintenance logs disclosed that this airplane was retrofitted with Garrett turboprops engine without a Supplemental Type Certificate (STC). There was no STC for the installation of this engine into this airplane.