Greetings from GoVacuum,
The bottom part of the bag, which you referred to as a "Boot" is actually called a Kirby G4 Mini Em-Tor with Fill Tube Assembly. Kirby Part # 199493S. The part is only available to purchase with the fill tube assembly, this is the part that the disposable paper bag connects to inside the cloth outer bag.
Kirby has redesigned this fill tube, as the original design sometimes would cause the inner fill tube to crack. The Mini Em-Tor Fill Tube Assembly, part 199493S comes already assembled with the "boot" at the bottom, and the vacuum bag collar (the part the bag actually attaches to) at the top of it. You will need to remove the cloth outer bag from your old part, and put it onto the new "Boot". The cloth bag connects the "boot" with a special zip tie, which is Kirby Vacuum Bag Clamp. Kirby Part # 196599S. Where it is possible to remove the bag clamp without breaking it, I will warn you that this is no easy task. Spare yourself the aggravation of having to try and remove the clamp, and just cut the bag clamp with a knife or scissors.
You will also have to remove the top of the bag collar at the top of the cloth bag by releasing the fill tube from the inside of the cloth bag.
All of the parts are located on our website, www.govacuum.com
. If you have any questions about the repair, you can get me on a live help, chat service located in the top right corner of our website. I'd be happy to walk you through this.