While I've never had the need to use them, some of my fellow SASS shooters say that they've found "hard to find" parts for SXS shotguns at
www.e-gunparts.com and
www.thegunrack.net (pulled these from SASS web forum).
While I'm also not a shotgun expert, I agree with Arcticap that the lock-up should be addressed first before the other issues. If it no lockie, it no workie.
Nice lookin' old shotgun!