After takeoff the aircraft was flown to another property some 25 kilometres away. As the aircraft was circled around the property the pilot noticed that the engine was not delivering full power. He changed the fuel tank selection but this did not restore the lost power. The aircraft gradually lost altitude and the pilot realised it would be necessary to land. On final approach at a height of about 30 feet above the ground the engine failed completely. The aircraft was landed heavily. An inspection of the engine found that it had seized. Earlier in the year the manufacturer had issued an Advisory Circular referring to engine overheating problems with the type and recommending that a modification be incorporated. The modification was to install a cooling fin to the engine to improve the flow of cooling air. The circular also stated that modification kits would be posted out to all owners. The owner/pilot had just received his modification kit but had not fitted it.