The pilot reported that while flying at 4,000 ft, engine operation began to deteriorate. Initially this was manifested by intermittent coughing/missing but over the next few minutes RPM and power output decreased to the point where altitude could no longer be maintained. Engine instrument indications were all normal and throttle movement or carburettor heat application did not rectify the problem. The engine finally lost all power and the pilot was committed to a forced landing. The aircraft finally came to rest in a rough rock strewn paddock, with substantial damage to the right main landing gear support structure. A subsequent inspection of the engine revealed no internal faults that could have contributed to the power loss. However, the engineer who conducted the engine strip had been associated with the aircraft for a number of years. He said that the engine has no oil cooler and tends to overheat especially on hot days or with too lean a mixture. The carburettor/associated fuel lines are located such that when the engine does overheat, the system is highly susceptible to fuel vapourisation. In the absence of any other obvious reason for the failure, it was his opinion that the power loss was due to fuel vapourisation.