While on a cross-country flight, the pilot was advised by radio of a severe thunderstorm and tornado watch along his route. At 1613 CDT, he was advised of a line of radar returns about 10 miles ahead. He replied that it "looks like a soft spot through there, just to the north of our position" and continued. At about 1621 CDT, the pilot declared an emergency. He reported that he had lost the engine (engine quit) and an aileron, and that the aircraft was down to 5000 ft. (The aircraft had been at 10,500 ft.) About 25 seconds later, he stated that the plane was hard to control, he was at 2500 ft and descending and he had lost his right wing. An examination of the wreckage indicated the plane had impacted in a flat spin. The empennage, right aileron and a portion of the outer wing panel (forward of the main spar) had separated and were not found.