Following a report of engine rough running a section of an exhaust valve was found to have broken away. A new cylinder assembly including an exhaust valve was fitted, but on take-off for a test flight the engine suddenly suffered a substantial loss of power. During the subsequent forced landing the aircraft ran through a fence and came to rest in a ditch. Two cylinder assemblies were found to have suffered internal damage and pieces of the missing section of the previously replaced exhaust valve were found within the induction system. Although the engine had been ground run by the engineers it had not been tested to full power. As the pilot was asked to limit taxiing and idling times to avoid overheating the new cylinder, engine run-up and pre-flight checks were conducted whilst taxiing. Take-off was commenced 230 metres along the 1900 metre runway and this was the first full power demand made on the engine since its repair. At about 250 feet agl a power loss was experienced, full flap was not used for the forced landing attempt and the aircraft struck the ditch which was 274 metres past the end of the runway.