The instructor and student were conducting crosswind training circuits on runway 07 at Redcliffe. At approximately 200 ft AGL after takeoff on the third circuit, the engine began to run very roughly. The instructor immediately assumed control and transmitted a Mayday call. A decision was then taken to carry out a landing, onto the clear swampy terrain, straight ahead. The aircraft nosed over soon after touching down onto the soft surface. Both occupants were able to exit the aircraft without assistance. The investigation found that the number 4 engine cylinder had a large fatigue crack in the non-finned base area, that extended approximately two thirds the circumference of the cylinder wall. No other defect was found with the engine, accessories or engine control systems that would have contributed to the rough running. It is probable that the rough running was caused by excessive valve clearances and/or piston to cylinder binding, as the crack opened under load.