Approximately 15 minutes after takeoff while maneuvering approximately 100 feet above ground level, the engine lost partial power or "bogged down." The pilot applied throttle; however, the engine did not respond. He scanned the engine gauges and no anomalies were noted. The pilot then applied carburetor heat; however, "did not have much time as we were about to land..." The pilot elected to perform a forced landing to a wet/muddy field. During the landing, the airplane impacted the terrain and nosed over. Examination of the engine revealed no evidence of preimpact failure or malfunction. The temperature and dew point at the time of the accident were conducive to serious icing at cruise power. Examination of the airplane revealed the vertical stabilizer and rudder were bent.