On August 21, 1993, at 0925 hours Pacific daylight time, a Beech C23, N6547R, experienced a partial loss of engine power while in cruise flight. The airplane was unable to maintain altitude and a forced landing was made to an open field, 30 miles west of Lakeview, Oregon. During the landing roll, the airplane collided with the rough terrain. Visual meteorological conditions prevailed at the time and a visual flight rules flight plan was filed. The airplane was substantially damaged and the certificated private pilot and her passenger were not injured. The flight had departed from Klamath Falls, Oregon, on August 21, 1993, at 0840 hours, and was en route to Boise, Idaho. During a telephone interview, the pilot reported that the flight was cruising at about 4500 feet AGL when the engine rpm dropped 300-400 rpm. Unsuccessful attempts were made to try to regain full power. The airplane was unable to maintain altitude and a forced landing was made to an open field. During the engine tear down inspection, it was found that the number one exhaust valve head broke away at the neck. There were no remains of the head present. The piston face was severely damaged, along with the bottom spark plug. Compression check of the number one cylinder indicated no compression. The valve stem remained in position in the guide. The broken end of the stem displayed severe heat damage and melting of the metal.