A loss or commanded reduction of engine power during the initial climb for reasons that could not be determined because of postaccident impact damage and fire destruction to engine systems and components. Also causal were the pilot's failure to maintain adequate airspeed and airplane control while attempting to return to the runway despite unpopulated, flat terrain immediately ahead that was suitable for an emergency landing; his decision to take off again with a known problem; and his lack of training in the make and model airplane.