Can you still boot games? I'm guessing so, since you said you FTP'ed over a new UDATA. Okay, here is what you should do. Load up the game and the (bad) installer. Just let it sit at the screen and don't do anything. As long as it's in the UnleashX screen, you will have FTP access. FTP from your computer and delete EVERYTHING in the C drive but DO NOT touch the xbox after it has been deleted. Then, use the C drive from an extracted SlaYer's disc and transfer it over.
You can find the entire C drive of a retail xbox on the SlaYer's disc in this directory:
Transfer everything from there to the Xbox's C drive. Then, go to your Xbox's E drive and transfer the following folder to your computer:
After that is backed up to your computer, delete the following directories on your xbox:
Then, FTP over brand new SC PAL installers (either UXE or Ndure) and try it again.