It sounds like there's a short somewhere between the stomp start switch and the back end of the starter where the brushes are located. Check all the connections at the battery and the starter, both hot and ground, and make sure you've got a cable disconnected at the battery as you install the new one. Gently touch the cable to the battery post before you install it to make sure there's not a short remaining. The problem is most likely internal in the old starter, but double-check everything, including the mechanical linkage and the return spring on the stomp pedal.
Jerry
