I know an OS 15 cv-r(x) carb fits and makes it run really good. You have to drill and tap a couple holes in the carb neck, but other than that, it was a really good upgrade on mine.
I did it because the carb on my engine was itty bitty and one needle to adjust only. I couldn't get that little bastard to run right. So, I pulled a carb off a dead OS, drilled/tapped a couple holes, then it purred like a kitten. Idle'd well and ran smooth from idle to WOT. Also gained quite a bit of grunt.