After conducting a thorough pre-flight inspection, the pilot prepared to ferry the aircraft to a maintenance organisation which was to perform a scheduled inspection. The aircraft performed normally until it reached a height of about 200 feet after take-off. At this point the engine lost a substantial amount of power and the pilot was committed to a forced landing. During the landing roll the aircraft collided with a fence and came to rest in the adjacent paddock. Initial inspection revealed that there was a serious leakage of fuel past the fuel filter bowl seal, and it was likely that air had entered the fuel system. The stirrup/clamp which held the filter bowl in place had been subjected to an unauthorised repair, which had probably reduced its overall strength. Fuel had been observed leaking from the filter bowl area several days previously, and it was possible that during the attempts to stop the leak, the stirrup had been incorrectly positioned. The fuel leak had not been drawn to the attention of a licenced engineer.