The airline transport pilot reported that while on final approach to land in his unregistered two-seat light sport airplane, when he retarded the throttle, the airplane’s engine experienced a complete loss of power. The pilot attempted to force land the airplane in a grass field, but the right wing struck a tree branch and the airplane subsequently impacted terrain coming to rest on its nose. The airplane sustained substantial damage to the right wing and fuselage. A Federal Aviation Administration inspector conducted a postaccident examination of the airplane. The fuel tank and rudder were not present with the wreckage and there were no airplane identification markings. Flight control continuity was established but an engine examination was not conducted. The inspector requested the pilot produce the operating handbook, operating limitations, and maintenance records for the aircraft, which he could not. The National Transportation Safety Board attempted to have the engine examined further but the pilot would not allow it. The pilot reported there was adequate automotive fuel onboard when the engine failed. He examined the fuel lines, the fuel filter, and the carburetor, and found no preaccident mechanical failures or malfunctions that would have precluded normal operation. The pilot reported that he did not feel carburetor ice was involved, but that the engine failure was a fuel issue. However, the pilot could not produce any evidence to confirm that.