You can always change your rep with any faction using a mod called the Fallout NV Cheat Terminal. I'm sure there's an easier way, but honestly, it's a fantastic mod that is one of my essentials. It should have way more than 1,900 endorsements. Whenever I come across any problem such as this in my game, there always seems to be a fix hidden in this somewhere. It's only one file and you can use the Mod Manager to download it - so if you'd like, you can go ahead and give it a try.