The pilot reported that while taxiing after landing the engine began to run roughly and stopped. He started the engine again and continued taxiing to the fuelling depot. After refuelling, the pilot carried out a fuel drain, removed and replaced the spark plugs, which were found to be clean, and started the engine. He completed an engine check and found that the engine ran roughly at idle but ran smoothly when operated at higher RPM. Shortly after takeoff, as the pilot was setting climb power, the engine failed. He was forced to carry out a landing on unsuitable terrain, and during the landing roll the nosegear broke off. An inspection of the engine found that the throttle to fuel control unit link rod assembly had become disconnected at the fuel control unit end following the loss of a split pin. This resulted in a mismatch between the internal position of the fuel control unit and the position of the throttle butterfly which was set by the pilot when he moved the throttle lever. The fuel control lever had apparently vibrated to a position where insufficient fuel was available to the engine to sustain engine power.