I have been very satisfied with Wolff Springs for the Colt 1911 and the Browning Hi Power. Also, Wolff's trigger conversion spring for the Beretta 92 is a phenomenal improvement.
Have you attempted to use a red 3M scotch brite pad on it? I cannot determine how deep the scratches on the slide are from the pictures. I have successfully removed minor scratches on stainless steel slides and frames this way.
I suggest you take it to the gunsmith at Marietta Firing Line. I believe his name is Andy. He can sort it out for you and then you can shoot it at the range there if you want to make 100% certain it is fully functional.
Greg Cote has the Metalform mags for $17.95:
https://gregcotellc.com/cart/1911-380-acp-colt-mustang-c-184/metalform-1911-colt-mk-iv-government-380-acp-7-rd-blued-380293-p-1549.html
Metalform made the mags for Colt with the M stamp on the base.
Bob's might have the other parts...
Definitely something a competent gunsmith could fix. If this is the correct Mossberg cross reference it appears Numrich still has parts: https://www.gunpartscorp.com/gun-manufacturer/mossberg/rifles-mossberg/472pca
There is also a Wolff Trigger conversion unit that is very good:
https://www.gunsprings.com/BERETTA/92, 96, AND CENTURION/cID1/mID2/dID36#65
I believe Wilson Combat uses it in their Beretta 92 Series pistols.
Since you have spoken with the owner, one option could be to get a written quote from another business that offers both cerakoting and gunsmithing such as GA Firing Line and see if the owner will reimburse you for any of all of the cost to restore it to the condition it was in when you first...
Try Andy at GA Firing Line. He does nice work. He double undercut and rounded the front of the trigger guard of my 19. Some of his other Glock frame work is on the website.
https://gafiringline.com/gfl-gunsmithing/pistol-services-2/