On February 18, 1994, about 1030 central standard time, a Cessna 150 airplane, N704CE, sustained substantial damage following a loss of engine power and forced landing in a field near La Crosse, Wisconsin. The certificated flight instructor and student pilot aboard were not injured. The local instructional flight operated under 14 CFR Part 91 without flight plan in visual meteorological conditions. The flight departed Winona, Minnesota, about 1000. In a statement made to the FAA, the flight instructor states he and his student were practicing steep turns when they experienced a complete loss of engine power. A forced landing was initiated onto a snow covered field. After touchdown the airplane nosed over and came to rest inverted. Postaccident examination of the engine revealed the exhaust valve in the number one cylinder had fractured.