It is just a guide plate for the brake shoes. You are only missing a corner and you still have the full length of that end of the plate. If you can weld the corner up and file it to shape, great. De-burr it at least and you could still use it. The fact that it is not any shorter than it was when it was new means it won't cause a failure of any kind as long as it is installed correctly. But since it is a brake part and all of us want the best condition brakes we could possibly have, I'd weld the corner up or replace it.
If you have trouble either finding a replacement or getting it welded up, send it to me and I will restore it for you and send it back same day. I would never charge to a do 4 second weld followed by a 3 minute clean up to shape. PM me if you want to do that.
EDIT: I just looked to see where you are... We are almost close enough to drive. My Aunt just moved out of Harvard ID. The mail would not take long at all...