This is a common problem with heavily-used older Sheridan rifles that have the soft front plug. You can move the metal back into place on the front plug with a ball peen hammer. I've watched Tim McMurray at MAC-1 do this. You softly close up the holes on each side with the hammer until it will hold the pin snugly. Not sure if it applies to this gun as the pin you have might be the screw type. There isn't much you can do about the body tube itself, but the plug can be manipulated enough to hold a roll pin. The gun is beyond a daily use example and the best you can hope for is getting it back to a point where it is an example of type.
If you have a tight fitting mandrel you can do the same to the holes in the body tube, but it takes a skillful hand.