On July 23, 2000, about 0815 Pacific daylight time, an unregistered Challenger II, registered to and operated by the non-certificated pilot as a 14 CFR Part 91 personal flight, collided with the terrain following a loss of engine power near Chiloquin, Oregon. Visual meteorological conditions prevailed and no flight plan was filed. The aircraft was substantially damaged and the pilot received serious injuries. The flight departed from the pilot's residence in Chiloquin, about 10 minutes prior to the accident and was en route to the Chiloquin Airport. The pilot reported to the Federal Aviation Administration Inspector from the Portland, Oregon, Flight Standards District Office that the engine had been running rough and that he had aborted two takeoffs prior to the flight. The pilot changed the spark plugs, then took off for the flight to Chiloquin. In a written statement, the pilot reported that approximately 10 minutes after departure, the "engine failed, and lost power over heavy timber terrain." The pilot spotted a clearing for a forced landing and made a left turn. The pilot reported that during the turn, the aircraft "...lost speed and went into a dive." The aircraft descended about 800 feet before the pilot was able to pull the nose up just prior to colliding with the ground. Information provided by the pilot revealed that he thought that the aircraft was classified as an ultralight and did not require an airworthiness certificate or registration. It was later determined that it did not meet the ultralight requirements as stated in 14 CFR Part 103 Ultralight Vehicles. The pilot also reported that the aircraft had not been inspected in accordance with an annual maintenance program. Approximately 263 hours had been accumulated on the engine at the time of the accident. The reason for the loss of engine power was not determined. The pilot at one time held a student pilot certificate, however, that had expired several years prior to the accident. No student endorsements were identified. The pilot reported that he had accumulated a total flight time in this make/model aircraft of 263 hours at the time of the accident.