Google, Gary Poh XS650. He makes offset sprockets for the XS. Typically he will tell you that with a 180 tire you will have to go out to a .750" offset front sprocket. He makes the .750 for 180/200 tire conversions. He has quite a few posts up at 650motorcycles.com showing pics of hardtailed XS's with a 200 rear. Keep in mind Gary says that anything over roughly .250 offset front will require you to space the outer alternator cover out to make room for the sprocket (he sells the spacer), you will also need to lengthen your clutch pushrod (he sells this also). To get his whole kit is roughly $500 bucks, but you get the sprocket, spacer for cases, pushrod peice, gaskets, and instructions. I had him machine and heat treat a .250 offset front for me to run a 160 rear tire. I might have to drop down to a 150.
Justin