The aircraft was being test flown following an engine top overhaul. Shortly after lift-off there was a sudden and substantial loss of power. The pilot landed straight ahead but was unable to prevent the aircraft from running off the end of the agricultural strip and colliding with a fence. An examination of the propellor revealed the failure of the pitch change actuating pin for the no. 3 blade. The aircraft logbooks indicated the no. 3 blade was damaged in a previous incident and subsequently repaired. The reason for the failure of the no. 3 pitch change actuating pin was not positively established. However, it is suspected the pin was damaged in the previous incident and the damage not detected during the repair procedure.