Hey mate,
I think stock its like 15 or 16?? maybe even 14, havn't checked.
Seriously though, for a pitbike i'd preffer a bit more low down grunt as fast 50s tracks are tight and you'll never get top speed on one, if ya go smaller u can tear outta the corners instead... The top speed on a dhz, pitpro, mso (anything with a stock 1p55 lifan) will be around 80 or 90, changing the front sprocket won't make it that much faster....
Anyway, thats just my opinion though.
Cheers.