The XR250 engine is no powerhouse. As a matter of fact, I'd put money on the fact that the CR engine (assuming it is a late model) makes at least 10-15 more rear wheel HP. In addition, the XR engine is not the most reliable engine when heavily modified. My dyno experience with XR250's has shown RWHP ranging from 14 all the way to 19HP, depending on conditions. Not exactly good in my book.
I would suggest a big bore kit and possibly a stroker crank for the CR engine. You could even add a flywheel weight to mellow it out some. That way you get the low end power you may be looking for, extra reliability and you will not have to spend a year fabricating.
Chris