I would think the removal method would be just as you said.
To find an exact replacment screw will be a problem. However, I am certain that the insert is some standard thread and you can probably find something close to original at a hardware store. A specialty screw and bolt store would be even better.
If I am not mistaken, that sigh set up has the screw in in the middle and the two on the outside is where positioning pins on the sight base would sit. The original sight had a thumb screw type deal thaat threaded into the center hole, and the pins on eithe ends of the sigh held the sight base in perfect allignment.
My point is that when you get the broken screw out, I don;t think there is a screw that would go into that hole. It would have been built into the original site that was made exclusively for the Bear bows that accepted them. If I am nottoo senile, I think they were called the "omni sights". These went along with the omni coupler on the stabelizer rods. IT was and entire system that I think was brought ot Beasr by either Jim Pickering or Owen Jefferies.
Hope rhis helps