The pilot was spraying a mature cotton crop adjacent to the company strip. After 20 minutes of flight, and shortly after commencing another spray run, the engine failed suddenly. The aircraft was manoeuvred towards an adjacent paddock with short soyabeans but it could not be aligned with the rows before landing. The wheels dug into the wet ground and the aircraft overturned. While skidding backwards the fibreglass back of the cockpit broke, allowing a large quantity of mud into the cockpit and making the pilot's exit difficult. He eventually used the fire extinguisher to smash a window to facilitate egress. Witnesses saw flame and black smoke emitting from the aircraft at the time of the engine failure. The aircraft operator found that the rear bearing of one magneto had become loose. This allowed the timing to advance and the engine backfired. An air inlet hose between the air filter and the carburettor was split and was sucked into the carburettor, thus stopping the engine. This occurrence was not the subject of an on-site investigation.