I re-drilled the CRF150R rear sprocket to match the EX500 bolt pattern. I used the sprocket carrier as a jig and used a transfer punch to mark where to drill. The new sprocket fit perfectly snug against centering faces on the interior diameter, and the two existing bolt holes I’m using in the sprocket are tapered, and thus self-centering. So I am fairly confident that my sprocket will be concentric, or at least within normal mass-produced tolerances. That being said, one of my new bolt holes was perceptibly off, and I had to remove a bit off one side with my die grinder. I could screw a bolt into the other three holes, but there was some drag and no wiggle as I caught the threads. Afterwards, I checked and could see faint witness marks from the threads on the side of my drilled holes, so they all got a light touch with the Dremel as well. Overall, it went pretty smoothly.
I failed to take a photo of the sprocket when I was done, but there’s not much to see. It looks like…a sprocket.
A bit of checking with a straightedge confirmed that I am a going to have to machine the face of the sprocket carrier to move the sprocket inboard at least 5-10mm. As it is, the chain would foul the frame tubes.