If you check the clock tower earlier in the game (where you see the intermission sequence showing the opening of the cave crammed with zombies), you will not have to get a spare generator for Isabela in Overtime mode, just ten Queens to stop Frank's zombification. If you wait until Overtime mode, at the point where you gather all the supplies for her, she will tell you that you must get the backup generator behind the clock tower before you can go after the Queens.