The white on top indicate a 'bashing'? As in it got dropped, landed on the sight and knocked it off?
You might be able to put the sight back into the dovetail. Put the barrel into a padded vise, the just put the sight back in, probably right to left(try it both ways if one doesn't work.), with a padded brass punch and plastic mallet.
If it won't go back, you can still shoot it using just the front sight. Sit the target on top of the blade just like you do a bead sight.
Mossberg does make ported, rifle sighted, barrels. Fairly standard thing for 'em. How to sight it in should be in the manual.
http://www.mossberg.com/wp-content/u...505-590-en.pdf