Perform a current draw test, while monitoring the current draw remove the fuses one at a time. When the current draw disappears check the devices that are protected by that fuse. As a mechanic with 20+ years of experience I have never replaced a starter motor for a current draw problem. I have replaced a number of alternators due to shorted diodes which can/will drain a battery. Begin with the current draw test, if after removing the fuses you are unable to eliminate the draw then remove the alternator wire. Let us know how the diagnosis develops.