During an aerial agricultural flight the engine began to lose power. The pilot observed a low oil pressure indication before the engine suddenly seized. During the subsequent forced landing, the pilot attempted to extend the glide in order to avoid trees. However, the aircraft stalled at a height of about 30ft and landed heavily on the left main landing gear, which then collapsed. An inspection revealed that two top cylinders of the radial engine had failed, and there was no significant oil left in the engine. The engine had accumulated only 10 minutes flight time since major maintenance had been carried out, during which a top cylinder had been changed. It was later found that the oil sump plug was loose. It was apparent that the engine oil had leaked from the engine, which had then seized. The LAME who performed the engine repair was apparently distracted by other events and had failed to tighten the sump plug.