You can fix using a PC with an IDE connection and CD/DVD drive and this rescue disk. This is a CD image. There are ways to do this from USB, but I haven't tried it. This should fix the dash and install an old version soft mod. You can always restore the Xbox to stock once you've fixed the dash.
Download Links:
Links are hidden from guests. Please register to be able to view these links.
You'll need XBOXHDM and to watch or read a tut on how to hotswap.
https://www.reddit.com/r/originalxbox/wiki/xboxhdm