Going back a few posts on the 19" rims, Maxiss tires are temporarily out of production, so your choice for 19" DT tires are the Goodyear/Dunlops. Period. Putting a 19" rim on the rear with a stock hub costs more than $100.00 for spokes. You will also need a rim and it has to be drilled for a 650 large diamter rear hub. Figure $250.00 for that. I personally have never used an Omars front wheel kit to put a front wheel (extremely narrow) on the back of an XS650, but comparing the physical size, amount of material and "Beef" that a 650 rear hub has, perhaps there is a reason the factory doesn't run a tiny, narrow front hub on the rear? Aslo with spoke flanges very narrow, the wheel under rider/bike weight and cornering loads if ridden aggressively, can over power a weak spoke triangle (narrow at the hub) very easily. This is done primarily for looks. Definitely not a racing application.
Get an aluminum Sportster rear hub and then you can have a spool with disc brake and get Sportster bolt pattern sprockets all day long. Did I say strong? Go to single row bearings and lose the stock, heavy double row bearings and you've got a real Pro set-up, not an appearence, wannabe wheel.
On the 17" rear wheel and tire availabilty thing, of course there are a billion 17" tire choices out there! Unfortunately that has little corelation to a bike who's chassis was design in 1968! Yes, you could get hubs, wheels, a swing arm, forks or whatever and eventually graft them to a 650 frame structure. Then what? Then you'll experience issues such as I mentioned where some serious re-engineering will be necessary, such as repositioning the motor or swingarm pivot or custom frame. And by the time you're all sorted out, your buddy put 18's front and rear on his bike and he's been riding everyday for the last 2 years.
When it comes to far-out custom changes, do some calculating, a bunch of sketches, mock fit-ups and lots of measuring before you start purchasing expensive components. You'll be miles ahead or at the least you may not end up way down an expensive dead-end path.