On October 3,1993, about 1230 central daylight time, a Cessna 150G airplane, N4646X, experienced loss of engine power during cruise and ditched in Truman Reservoir near Osceola, Missouri. The solo private pilot reported no injury. The airplane was substantially damaged. Visual meteorological conditions existed in the vicinity. The personal flight originated in Belton, Missouri about 1145 without a flight plan and operated under 14 CFR 91. The pilot stated the engine made a bang, shook and ran roughly about 5 miles from destination. As he checked indicators, the engine quit running. The airplane glided to water touchdown about 60 yards offshore. The pilot exited the submerged cabin and swam to shore. External examination of the engine showed the number four cylinder fractured above the cooling fins at its cylinder head. Internally, the number four cylinder head was found fractured at the piston pin boss.