First, check for a short from the motor casing to the motor wires using a DVM.

Next, try placing a low-amp (@ 1-2 amps) fuse in the positive (red) battery wire to the new ESC. Run the motor without a prop, and you should be able to go to full throttle without blowing the fuse. Measure the current consumed while running with no load. It ought to be close to your motor's specs if it's still in good shape. Adjust the fuse rating to be just over your motor's specified idling current. Radio Shack will probably have the fuses. I think you really need a dynamic test to tell for sure, and if it's really shorted, the fuse will protect the ESC. Good luck with the troubleshooting.