The XR250 rim will likely have a 520 chain on it, so you'll need to change the front sprocket to match. JT makes a 520 countershaft sprocket that will work on your 200 engine, but please measure the splines to confirm. Here's the JT sprocket:
http://www.jtsprockets.com/catalogue/sprocket/JTF328
You can order the sprocket from a local motorcycle shop or buy it on eBay.