On January 14, 2000, at 1530 central standard time, a Cessna 150F single-engine airplane, N6691F, struck the terrain during a forced landing following a loss of engine power near Livingston, Texas. The airplane was owned and operated by the pilot under 14 Code of Federal Regulations Part 91. The commercial pilot was not injured, and the airplane sustained substantial damage. Visual meteorological conditions prevailed for the local aerial observation flight, and a flight plan was not filed. The flight departed the Livingston Municipal Airport at 1415. During a telephone interview, the pilot reported to the NTSB investigator-in-charge (IIC) that he was flying for a contractor with the Texas Forestry Service to locate fires. The pilot had flown the airplane for 3 1/2 hours earlier in the day and no discrepancies were noted. He had landed the airplane at the Livingston Municipal Airport for refueling, and thirty gallons of fuel were added to the long range fuel tanks. After takeoff, approximately 20 miles east of Livingston, at an altitude of 2,500 feet msl, the engine started "running rough, knocking, and rattling." A field was selected for the forced landing. Final approach airspeed was 40 mph for the short field landing over trees. While the pilot maneuvered the airplane to avoid the last tree, the airplane impacted the ground "hard." The nose landing gear collapsed, and the airplane nosed over to the inverted position. The FAA inspector, responding to the site, found structural damage to the engine firewall and both wing spars. Trees at the approach end of the landing area were estimated to be 120 feet high. On January 26, 2000, the FAA inspector and the manufacturer representative examined the Continental O-200-A engine (serial number 63019-6-A). The #1 cylinder exhaust valve head was found separated from the valve stem. Impact marks on the piston head were consistent with the valve head penetrating the piston. Damage to the exhaust valve precluded a determination of the cause of the valve failure. At the time of the accident, the tachometer reading was 4,729.7 hours. Maintenance records revealed that the engine had accumulated 743.7 hours since the last overhaul dated July 1994. Time of engine operation since the last 100 hour inspection was 102.7 hours.