If you still have the old motherboard you can swap the EEPROM onto the new motherboard (desolder both chips, then solder the old EEPROM to the new board).
However, perhaps you can avoid this. Is this the retail hard drive that came with the xbox, or is it a new one you've put in? If you've unlocked then locked it with ConfigMagic, the master password will be TEAMASSEMBLY. Also you can try this: http://forums.xbox-scene.com/index.php?showtopic=174024
Finally, if you have a backup of C and E on your PC, look to see if there is a backup file of the EEPROM. I think that using this file, LiveInfo can generate the unlock code.