On September 18, 2004, at 1600 eastern daylight, an amateur-built Abbott GTX gyrocopter, N702BX, sustained substantial damage during a forced landing after takeoff from runway 27 (5,000 feet by 100 feet, asphalt) at the Oakland County International Airport (PTK), Pontiac, Michigan. The private pilot was not injured. The 14 CFR Part 91 personal flight was departing on a local flight. Visual meteorological conditions prevailed. No flight plan was filed. Prior to takeoff, the pilot had performed a 30-40 minute ground run to check the engine cooling system that had been modified prior to the flight. The pilot reported that the engine operating temperature during the ground run was 140 degrees F and there were no coolant leaks. The pilot reported that during the takeoff, the gyrocopter's climb performance was not as good as he had anticipated. By the end of runway 27, the gyrocopter had climbed to about 200 feet above ground level (agl). He reported the engine started to lose power so he declared an emergency and attempted to return to PTK. He reported that the gyrocopter "did not have enough altitude or speed to make a proper landing." The pilot landed the gyrocopter in an easterly heading in a grassy area at the end of runway 27. A Federal Aviation Administration (FAA) airworthiness inspector examined the gyrocopter. The inspection revealed there were no defects to the fuel, fuel filters, fuel pumps, and carburetor linkage. A compression test revealed that the aft cylinders exhibited "little compression capability." A visual inspection of the cylinders with a borescope revealed that both aft cylinders exhibited evidence of scoring and overheating. The FAA inspector reported that the gyrocopter pilot/owner had altered the engine cooling system by moving the radiator approximately four inches aft of its original location, because the engine operating temperature was higher than normal (190-200 degrees F). By moving the radiator aft, it moved the radiator further away from the engine exhaust system, and it provided for increased cooling system fluid capacity by lengthening the radiator hoses. The FAA inspector reported that the pilot/owner was unable to return all the coolant that he had drained from the radiator prior to the modification, back into the radiator after the modification was made. The FAA inspector reported that the cooling system modification was made at the advice of the airframe kit manufacturer, however, the pilot/owner was not a mechanic and he did not have the required cooling system service instructions available at the time the modification was made.