On January 28, 1998, at 1550 Pacific standard time, a homebuilt Raser Mini 500, N500YY, operated by the pilot as a 14 CFR Part 91 personal flight, collapsed the right landing skid and rolled over after initiating an autorotation as a result of a loss of engine power shortly after takeoff from Sportsman Airpark, Newberg, Oregon. Visual meteorological conditions prevailed at the time and no flight plan was filed. The helicopter was substantially damaged and the airline transport pilot, the sole occupant, was not injured. During an interview with a Federal Aviation Administration Inspector and subsequent written statement, the pilot reported that he had just lifted off and attained an altitude of about 20 feet, when the engine experienced a loss of power. The pilot initiated an autorotation near the south end of the airpark to a plowed muddy area. The pilot stated that the helicopter was yawed slightly to the left on touchdown. The right side landing skid collapsed and the helicopter rolled over onto its right side. Inspection of the engine revealed that one of the two cylinders would not hold compression. Further inspection of the cylinder found evidence of overheating and damage to the rings on the piston. The pilot reported that the engine had been experiencing heating problems, and that he had also modified the carburetor.