Franklin
The sight should be "silver soldered" on . If you cannot do this , I suggest finding a good gunsmith. You want to get it back on in the same place it was, or else you will have windage problems when you shoot. I silver solder sights on Civil War muskets quite a bit, and it is very strong. It is a little tricky. You can get silver solder at a good hardware store, alon with the flux it needs. You will have to rough up the area under the sight to get a good solder bond. I use paste flux.
Clean the base of the sight. Rough it up with 320 grit sandpaer
Clean the bbl wear the sight belongs. Use 320 grit sand paper, to rough it up.
Apply paste flux top area. find something to hold the sight level on bbl. You might be able to use a small C clamp. If possible level the front sight with the rear, this will keep them in line with each other. This is the tricky part.
After all this apply the torch heat to the sight base, and put a drop of solder at the base of the sight. when the heat is right the solder will flow under the sight base, and form a strong bond.
I hope this helps, and you can understand it the way I wrote it.
Rebel Dave