ETHAN, S.D. — A tractor is more than an old machine sitting in a shed to Austin Miller. It can be a piece of family history, a mechanical puzzle or a reminder of farming before computers and screens became part of nearly every piece of equipment. Miller, 31, of Ethan, has collected about a dozen vintage tractors, most kept inside sheds and Quonset buildings. Nearly all are running, and many connect to his family, neighbors or agricultural histor…