If you are a reloader; using some clean reclaimed shot may give you the patterns sought. These days, it's always a good idea to pour it out in, say a large stainless bowl not to be used for food and double check it w/a good strong magnet before using it. Steel shot shows up from time to time in 'some' reclaimed, in spite of efforts to eliminate it by the seller(s). If you are not a reloader, it will be worth the effort to make friends w/some skeet shooters that are or look into going there yourself w/new or used equipment. Components are scarce at present, but there are still savings to be had reloading vs. new and for your quest that may be your best bet w/o the need to modify your interesting and valuable [to the right person(s)] shotgun. I knew of serial number 001 in an Iver Johnson 28 ga. skeeter at one time. It was a nice well used but not abused gun; kept up with it thru three owners, but lost track after the third died and never knew whot became of it. Sounds like a very nice gun that you have! Ultimately, the decision is yours, but once modified it won't be original. You have to decide if that really matters... to your personal enjoyment. That's your answer. Should you decide to have it opened, get someone who knows what they are doing to perform the work and never look back, aside from verifying that it meets your need(s) on a grease plate. Don't over think it. How do you shoot it now? That's likely the best starting point. If you are going to use it for rough shooting on dove or feral pigeons or wish to perhaps try some flyers or helice in a 28 ga race, I would definitely NOT modify it by opening the chokes! Just my thot's.
