At approximately 1640 Mountain time, the airplane impacted terrain, seriously injuring the Private pilot, who was the sole occupant. The aircraft was destroyed. The IFR cross-country flight, which departed Jackson Hole, Wyo., about two and one-half hours prior to the accident, was in an area of IMC at the time the accident sequence began. According to audio tapes recorded at Seattle Center, the pilot was in cruise flight when he reported that he could not maintain altitude because the aircrafts engine was not producing enough power. He was assigned a lower altitude, but the engine continued losing power. Soon thereafter, he reported the engine had quit and radar and radio contact were lost, and he reportedly continued his descent into a mountainous valley, where he attempted to execute a controlled crash into the terrain.