The pilot and passenger departed for a local flight to practice landings. At about 200 to 300 feet AGL on the crosswind leg, the engine quit. An attempt to restart the engine failed and the pilot landed the aircraft in a pasture which was sloped, rough and had a dry creek bed in it. Upon reaching the creek bed, the airplane nosed over. Examination of the aircraft revealed that the carburetor mixture control linkage was frozen and would not respond to movement of the cockpit control. Disassembly of the carburetor revealed evidence of moisture and water contamination, accumulated over an extended period of time, which caused rust, corrosion and binding of the mixture control shaft in the lean to near idle cutoff position. Examination of the engine records revealed that the last time the carburetor was overhauled was about June 2, 1983, while installed in an experimental aircraft. The engine was installed in the accident aircraft on May 17, 1991, after accumulating about 132 hours in about 8 years and 6 months. The manufacturer recommends that the carburetor be overhauled every 5 calendar years.