The MJD44E2 in the schematic is a darlington transistor, 80V, 10A, 20W, which a good match with the 2N6667, but not the TIP42, which is a 6A, non-darlington device. The much higher Hfe (gain) of the darlington device might needed by the circuit.
Does radio shack have any darlington PNP transistors, 80V, 10A, 20W?
BTW, the schematic appears to be confidential to Hobart. In the future, post only a snippet of the schematic so that it is useful for the purpose, but not identifiable for any peticular product, or application that is proprietary in nature.