On August 1, 2012, about 1120 central daylight time, a Beech H35 airplane, N4684D, sustained substantial damage when it struck trees following a loss of engine power near Holly Lake Ranch, Texas. The pilot received serious injuries. The aircraft was registered to an individual and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a business flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The flight originated from the Wood County Airport (JDD), Mineola, Texas, about 1100, and was destined for the Gilmer Municipal Airport (JXI), Gilmer, Texas. The pilot reported that he performed a pre-flight run-up with no anomalies noted. He then departed JDD and climbed to 7,500 feet altitude en route to JXI. During the descent to JXI, at 6,500 feet altitude, the engine experienced a total loss of engine power. The pilot reported that he turned the airplane toward the Holly Lake Ranch Airport (16TE), and attempted an engine restart to no avail. The airplane struck trees off the end of runway 27 at 16TE. Examination of the airplane’s engine after recovery from the accident scene revealed that the idler gear pin was missing from the rear of the engine, and both nuts and washers were missing from the idler gear pin attachment studs. The idler gear pin was found in a plastic trash bag that contained smaller parts recovered from the wreckage site. The idler gear had fallen in to the rear crankshaft gear and damaged the rear crankshaft gear teeth. The nut was missing from the forward left side lower through bolt on the crankcase half. The nut was also missing from the forward left side engine mount bolt. Examination of the maintenance records revealed a piece of paper attached to the log book indicating that the engine was disassembled and inspected after a propeller strike. The crankshaft was turned 0.010 inches undersize and the engine was re assembled. The entry was dated July 25, 2012 and was signed by the pilot, who also held a mechanic certificate with airframe and powerplant ratings issued by the Federal Aviation Administration. According to manufacturer drawings, the aft crankshaft gear engaged the idler gear which in turn engaged both magneto drive gears. Without the idler gear in place the magnetos would cease to operate.