The airplane sustained substantial damage when it impacted terrain during a forced landing following a loss of engine power on downwind at approximately 1140 Mountain time. The Private pilot, who was the sole occupant, was not injured; visual conditions prevailed. Just prior to turning right base and at 1000 feet agl, the engine lost partial power. The pilot turned toward the runway but attempted a forced landing to a nearby road. During the forced landing, the airplane landed in a ditch approximately 20 feet from the road. The airplanes right wing struck a fence post, and the airplane came to rest upright. The pilot reported the right wing was bent, the right main landing gear was separated, and the nose gear was collapsed.