On May 6, 2000, about 1330 Eastern Daylight Time, a Cessna 120, N2842N, was substantially damaged during a precautionary landing near Beaumont, Pennsylvania. The certificated private pilot and passenger were not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the personal flight conducted under 14 CFR Part 91. The flight departed Bloomsburg Municipal Airport, Bloomsburg, Pennsylvania, about 1300; and was destined for Skyhaven Airport, Tunkhannock, Pennsylvania. The pilot stated that he was in cruise flight, about 3,300 feet above mean-sea-level. He observed a decrease in oil pressure, followed by an increase in oil temperature. The engine began to run rough, and the pilot performed a precautionary landing to a field. During the landing, the airplane struck uneven terrain, and came to rest inverted. The propeller, cowling, firewall, and horizontal stabilizer sustained substantial damage. The pilot further stated that the engine was originally manufactured in 1947. It had an overhaul after approximately 950 hours of operation, in 1961. The pilot added that the overhaul occurred before he purchased the airplane, and the reason for the overhaul was not documented in the airplane logbooks. At the time of the accident, the engine had accumulated approximately 1,778 hours of operation since the overhaul in 1961. Examination of the wreckage by a Federal Aviation Administration Inspector revealed oil inside the cowling. The Inspector was not able to determine the source of the oil leak. The pilot planned to have a mechanic further examine the engine. About 1 month after the accident, a mechanic observed a 2 to 3 inch crack in the crankcase. The crack was in the vicinity of the "number one" cylinder. The mechanic said the he could not be positive the crack occurred before the impact, but that it would explain the loss of oil pressure, gain in oil temperature, and partial power loss.