Please don't take this as a flame, because it isn't.
I've always wondered about the concept that conversions to readily available calibres is undesirable. IMHO, it makes far more sense to convert a mixmaster to a cartridge that is easy to obtain and basically similar in most aspects than it is to shoot out a collectable original.
Go ahead, they're your rifles after all but most of us don't have ready access to the "spares bin". For many of us, the firearms that are in original state are quite hard to come by. Right now there seems to be a plethora of milsurps available. Some are decently priced, some aren't. Most are mixmasters and I suspect the last of what the nations of the world are willing to dispose of.
This is a great time for milsurp enthusiasts. The availability of Mausers, Lee Enfields etc is fantastic. If only the money for these was also available. In Canada, ammunition has been a problem as well but that's just politics and an overly enthusiastic bureaucracy.
A year or so ago, there was an outfit in Canada selling 7.63, Swiss1906/29 parabellums. The price was on the high side but the pistols were listed as excellent. They are also restricted only and not on our prohib list. So an opportunity for someone without the proper license to obtain a "Luger".
Recently there have been a couple of other outfits selling similar pistols, one is slightly more expensive than the other but quality is the same. I wouldn't shoot one of these pistols. They are just to clean. I would shoot a mixmaster in 9mm though. Where do we draw the line? I guess it's a personal choice.