I dug around for a month looking for a "bolt on kit" way of doing just this, ie., updated modern caliper and/or rotor on an otherwise stock rear end. Retiredgent's solution is probably as good as anything else, and simple to boot.
Robert at Omar's has a not-well-advertised-on-his-website underslung Brembo bracket:
Shown with his EBC rotor mounted on a front hub (19" conversion kit), requires a stay (not installed in pic), but you get the idea...
I didn't like the look of the caliper bracket stay/link, and preferred a smaller rotor, so I went another direction:
Again, on a front hub, but can't see why you couldn't use it on a rear hub if you really wanted to... The rotor carrier was quite a piece of work, but the rotor, caliper, and caliper bracket are all inexpensive late-model Ducati/Aprilia salvage pieces... and no brake stay!
In any event, putting the caliper below the swingarm gets you out of the way of the shock, with a little more room for creative stay mounting, line routing, etc. Take a look at the various RD/RZ rear caliper brackets; several of them look they would bolt-on to the stock XS rear, and might open up a bunch of caliper options.