On October 5, 2002, about 1300 Pacific daylight time, an unregistered homebuilt Quicksilver Challenger II airplane made a forced landing following a loss of engine power during takeoff from Porterville, California. The airplane was owned and operated by the noncertificated pilot under the provisions of 14 CFR Part 91. The airplane sustained substantial damage. The pilot, the sole occupant, was not injured. Day visual meteorological conditions prevailed and no flight plan had been filed. During a telephone interview, the pilot stated that he had climbed the airplane to 300 feet and entered a right bank when the loss of power occurred. He made a forced landing in a vacant field next to the airport. The left main landing gear separated after coming into contact with a tree. He turned the engine off as he exited the airplane. The pilot was not aware that the airplane did not comply with the requirements of 14 CFR Part 103, which specifies the requirements an aircraft must meet to operate as an ultralight vehicle rather than an airplane. The pilot had approximately 50 hours flight time in airplanes.