On June 8, 2003, about 1600 central daylight time, an amateur built experimental Sesin EXEC90, N8246J, operated and piloted by a student pilot, sustained substantial damage when it impacted terrain during autorotation near Searcy, Arkansas. Visual meteorological conditions prevailed at the time of the accident. The pilot reported no injuries. The 14 CFR Part 91 personal flight was not operating on a flight plan. The local flight departed from a private airstrip four miles east of Searcy, Arkansas, about 1515. The pilot reported he landed the helicopter after the water temperature gauge indicated an over-temperature condition. The pilot stated he saw the belt to the water pump had slipped off the pulley. He put the belt back on the pulley and ran up the engine while confirming engine temperatures were within limits. He hovered the helicopter for approximately three to four minutes and did not observe any problems with the engine temperature. The pilot stated he had completed a normal takeoff and climbed to 300 feet above ground level (agl) when he "noticed the water temperature rising rapidly." The helicopter "immediately" began loosing power, and he increased the engine throttle and the collective to maintain altitude. The pilot reported the engine failed a few seconds later and he began an autorotation. He veered to the northwest towards a clearing and began flaring 15 to 20 feet agl. The nose of the helicopter yawed right and he did not have enough tail rotor authority to control the helicopter. The helicopter hit the ground, yawed 45 degrees to the right, and rolled over onto its left side.