The non-certificated pilot reported that he departed his private airstrip to the north to "ride over his farm." He added that he noticed that the engine was "running strange as if it had water in the fuel." During the initial takeoff climb, the engine lost power while climbing through 300 feet. The best suitable forced landing was a cultivated rice field, so the pilot attempted to land the tailwheel equipped airplane in the field. During the landing roll on the soft terrain, the airplane nosed over and came to rest in the inverted position. The pilot was assisted out of the airplane by his grandson, who witnessed the accident. Examination of the airplane revealed that the airplane sustained structural damage. The reason for the reported loss of engine power could not be determined. Weather was reported as clear skies with light and variable winds.