I've noticed there is a large gap in the range of available tooth counts, I haven't found anything between 18 and 72.
I noticed that also, they may have to be custom made if you wanted a 50t or 60t or similar. I think the reason is that Kart riders only need certain sizes and anything else wouldn't sell so it doesn't get made.
just to think out loud with some numbers...
using astro 3210 10 turn - 135 Kv
at 48volt = 6480 rpm
if you wanted to go almost 35mph you'd need 450 rpm for 26" wheel.
thats 14.4 : 1 ratio
or 2 stages at about 3.8 : 1 each
12t - 72t = 6 : 1 ratio, may be noisy with 12t
18t - 72t = 4 : 1 ratio, which may be a bit quieter
I guess my tentative vote then is for an 18t 8mm bore sprocket
I was considering a 3/4 bore sprocket from here http://www.gokartgalaxy.com/engine_sprockets1.htm and machine a bore reducer, but if Gary has a sprocket made 18t with 8mm bore I would definitely buy from him.
Here's a page with some sizes, and they have 66t listed as the smallest available http://www.pegasusautoracing.com/kart-sprocket-pitch-diameter.asp , but when you go to their sprocket selection 72t is the smallest they stock.
edit: duh, I forgot to factor in the geared hub, so I probably would want the 12t
I guess the more sizes available the better