The glitch, on the Ruger Mk pistols, the receiver is the Serial # part, and it's a PITA to have to change a barrel out on it.
Hence after you manage to spin a barrel off it (without mar'g anything), you have to clean up the receiver threads, then have to set the new barrel shoulder back to get it to index correctly.
If you go to Clark's, you can pick up a pull down Ruger barrel for around $35 and send it to to have it threaded for around $40. But you only into it for half the battle, being that you still have to have someone spin off the old barrel and install the new barrel on.
As for replacement barrels, hate to say it, but the Ruger Mk target barrels are able to produce tight groups, while with a after market barrel, you playing luck of the draw instead. If your looking for an after market barrel that will print tight, then your talking Volquartsen or Clark, and your going to be into the barrel for a few hundred, then another hundred to have it installed on your receiver. Yes, you can get something like a Pac-lite receiver with barrel for a few hundred, but it not going to hold a candle to what a Ruger target barrel or a target Clark/Volquartsen will group instead.
Trust me, I have a factory target MK II with Oem slab side barrel that will eat the same hole for a hundred rounds until it has to be cleaned, and not way in hell that I would change that barrel out to something that will not shoot as tight instead.
So unless you have shot the barrel out on the 22/45, then it behooves you to replace it with a another barrel/ barrel upper that may not shoot at tight instead. If you can not find a smith that will not charge an arm and leg to just shorten, thread, and set the front sight back on the barrel you have, then just send the pistol into Ruger to have them change the barrel out on it with one of their threaded barrels isntead.
To add, this is the Gunsmithing forum, and not the Build forum, So if you just asked to begin with, pretty sure that someone local to you would have already threaded your barrel for you, and would have cost you a pizza and 6 pack only instead. If you where local to me, since I have some free time right now, it would have been a done deal already.