I take it that the small round object is the part that has come off, and that fits into the slot. Correct? I've done a search for T Slot bolts and have found a large number of bolts, but none quite as long as you require, and many that might fit with a little filing down. If I'm seeing this right, the knob moves down the threads to tighten things up. So in a pinch, you might be able to find a workable bold and file the head down to the correct thickness.
What about having the head spot welded in place?
Home Depot and Lowes have a few drawers of specialty bolts that you might check. If you don't mind grinding the head down, you might just find something that will work. Seems like length is the real issue.
The more I do, the less I accomplish.