The radial engine lost power during a full-lenght takeoff from the 5,017-foot long concrete runway. The 9,957-hour airline transport pilot was scheduled to perform a local aerial application flight. The pilot reported that the tailwheel-equipped airplane became airborne about the midfield point of the runway. As the airplane neared the departure end of the runway, while at an altitude of 100 feet agl, "engine roughness began" and the airplane was unable to maintain altitude. The pilot held the best climb airspeed and elected to make a controlled descent into small trees located off the departure end of the runway. The airplane nosed over, and came to rest in the inverted position. There was no fire. An FAA inspector examined the airplane and reported there was sufficient fuel on board for the flight and both water accumulators were empty. The airplane's last annual inspection was performed on March 21, 2007. At the time of inspection, the airplane had accrued a total of 7,392 hours. The 450-horsepower radial engine had accumulated 7,500 total hours and 100 hours since it was last inspected. The pilot calculated that weight of the airplane during takeoff was 4,330 pounds, 169 pounds below takeoff gross weight. He also calculated that the density altitude at the time of the mishap was 4,400 feet. The reason for the partial loss of engine power could not be determined.