the smallest using regular bearings is 13t. The old MT Racer had a 12 T cb (part number A812) but you need to go to a needle bearing on the cb. There's a post in here somewhere I mentioned it to someone, maybe has part numbers.

What spur are you running? 52 is biggest that I know of. If you have a 49 on there now you'll notice better acceleration with the 52 for sure. My MT2 is pretty fast for me with 13/52 even though thats a slow ratio. I ran with a guys Savage with 18/47 and I didnt get smoked all that bad with my OS .18 cvrx.