On November 6, 1994, about 1500 central standard time, a Cessna 172, N734LJ, piloted by the private pilot/registered owner, experienced a loss of engine power during cruise flight and made a forced landing in the vicinity of Hudson, Wisconsin. The airplane sustained substantial damage when it nosed over during the landing roll. The pilot and the two passengers on board the airplane reported minor injuries. Visual meteorological conditions prevailed for the local flight, no flight plan was filed. The flight operated under 14 CFR Part 91, and originated from Lake Elmo, Minnesota, approximately 1400. The pilot stated he was in cruise flight at 2,500 feet mean sea level (MSL) when he felt a vibration in the airplane. The vibration increased in intensity until the engine suddenly stopped. The pilot reported he was unable to restart the engine, so he made a forced landing in a freshly plowed field. He stated the terrain was soft and the airplane nosed over during the landing roll. A postaccident engine teardown was conducted. FAA Inspectors reported severe damage to crankcase, valves and cylinders. They stated it appeared the #4 cylinder exhaust valve keeper was displaced and the "...valve caused interference and subsequent engine stoppage." The airplane had flown about 6 hours since the most recent annual inspection, and about 77 hours since major overhaul of the engine.