During a test flight following an engine overhaul, the pilot reported that everything was normal for the start, taxi, takeoff and climb to 2,000 feet AGL. A slight roughness was noted and corrected with a richer mixture. A short time later, the engine began to run rough again and a full rich mixture was required for normal operation. The flight was returning to the airport when the engine roughness increased until there was a total power loss. Unable to reach the runway, the pilot initiated a forced landing to a wheat field. During the landing roll, the airplane collided with the terrain. During the post-crash investigation, the pilot reported that this engine had experienced several power losses over the past 28 hours flight time since the airplane was built, with no definite reason found as to the cause of the problems. The pilot stated that he was going to replace the engine with another and was not going to have the accident engine inspected.