The purpose of the flight was to "shakedown" the aircraft and to break in a newly overhauled engine. A thorough preflight was made and some water was drained from the fuel tanks. (The aircraft had not been flown for some period of time.) While climbing after takeoff, the cylinder head temperature (CHT) was noted at 550 deg Fahrenheit and climbing slowly. The pilot leveled at 700 ft and throttled back, and the CHT stabilized at about 555 deg. About 1 minute later, the engine began running rough, and subsequently, there was a considerable power loss. The pilot selected a field for a forced landing. But on final approach, he noticed that it was freshly plowed and rough. An S-turn was made to land on a gravel road. The aircraft was still in a bank during touchdown. While on the landing roll, the pilot was unable to keep the aircraft on the road with a 6-8 kt crosswind from the right. The plane collided with a fence. An investigation revealed that the No 1 exhaust valve rocker arm, PN 639615, had failed.