I also have a CR250R, 1997. I recently changed to a 51 tooth rear and left the front sprocket stock (14T I think?).
I also put on a new O-ring chain while I had it all apart.
I bought everything trhu a local dealer, and his price was as good as ebay. I really like to support the local guys when possible.
Although the sprocket made a small difference in the low speed ridability, the biggest improvements were felt when I put in a 14oz flywheel weight and a recent Gnarly FMF pipe. The bike really crawls around the woods now with much smoother power delivery.
regards, Greg