Whichever version, adjustable or fixed sight, I think a barrel from a 45 caliber Vaquero or Blackhawk might well be made to fit, Ruger wasnt prone to use bores that weren't cut with the same tools. The Blackhawk or Vaquero barrel would have to be turned and threaded for the longer frame of the ROA, but the sights would match, and the lever catch lug would need to be installed. You would end up with the ejector housing screw hole, but it could be filled with a cut off screw..
Flint