The pilot was engaged in the spreading of urea. During the operation he had observed that the aircraft was not performing as well as normal. He carried out a trouble check and after the completion of some rectifications and an engine run, believed he had rectified the problem. Six more sorties were completed without problem. However, shortly after takeoff on the next sortie the engine again partially lost power. The pilot decided to return and land, but not dump the load. During the turn toward the airstrip the aircraft stalled and impacted the ground with the left wingtip. The investigation determined that the loss of engine power was caused by a faulty magneto and contributed to by the generally poor condition of the engine. The engine had not been operating normally for some time, and attempts to rectify the problems, by unlicenced maintenance personnel, had not been successful.