The aircraft had not been flown and had stood in the open for about two months. Some 20 minutes after DEPARTURE the engine commenced to run roughly, and the pilot elected to divert to the nearest suitable aerodrome. Shortly afterwards, the engine backfired severely and black smoke entered the cabin through an air vent. The engine subsequently stopped completely and the pilot was committed to a forced landing on unsuitable terrain. The gear collapsed and the aircraft collided with two fences before coming to rest. On vacating the aircraft, the pilot discovered that a fire was burning under the cowls on the right side of the engine. The fire was extinguished by the pilot and passengers. The cause of the engine failure could not be determined. However, the insulation of both magneto coils was badly deteriorated. It is considered likely that during the period that the aircraft was inactive, moisture accumulated in the magnetoes. This moisture, together with the deteriorated insulation, probably caused the magneto to misfire and fail. The fire developed in the engine bay after severe backfiring caused the exhaust cluster on the right hand side of the engine to separate from the muffler. The hot exhaust gases then ignited the fibreglass cowling and the melting resin emitted the black smoke which entered the cabin via the hot air vent.