The Ofna single speed spur setup uses the 51T spurs:
http://www.rc-monster.com/proddetail.php?prod=ofn30110
Luckily, I happen to have a few from the old days when using an emaxx tranny with strobe slipper setup was common.
Note about the Ofna single speed kit: It does not come with support bearings or outdrive cups so you need to get those seperate. Also, my kit did not come with the needed grub screw, which is 5mm.