I don't have a Spec. catalog here, but I'm pretty sure that those bikes have standard BMX cranks and sprockets. So any sprocket from your local bike shop or Dan's Comp should work. Just choose a size and color and go for it. If you go to a bigger front though, remember that you'll probably need a longer chain, depending on where your axle sits in the dropouts. If there's room to use the same chain go for it because a short rear end is best. I wouldn't go too much bigger in one step in terms of size. It can get hard to pedal pretty quick. Another option that can be a little more tricky would be to go smaller in the rear. A friend of mine figured out that by swapping out the bearings and loosing a spacer, you can fit an Odessey BMX one piece 9 tooth driver on the stock single speed hub. No promises, but it worked on his Woodman and my Novara, which look to have been made by the same factory in Taiwan as the Spec.