I learned the part numbers of parts by looking at the lists in suppliers' catalogs. The lists for transistors are catagorized in order of voltage and current ratings. The prices are also shown.
600mA might be the average current of a DC motor but its max current might be 3A. Then if 40V is high enough I would use a TIP31 NPN power transistor that is rated for 3A and a 2N4401 to drive it.