Chain and sprocket question

[250sxman]

The bike I have needed a new sprocket when I got it. It had a 47 tooth aluminum rear sprocket, adn the only one I could find to fit was a 40 tooth rear. Well I got it today and went to install and my chain was way too long. I know the two sprockets are different diameters, and I definitly need a new chain, but how do I know what chain to buy? The only numbers I can find on the old chain are 520. I counted the links and there are 106.

[thor22racing]

what kinda bike is this on?

[dirtjunkie85]

I'm assuming this is going on a 250sx as your name suggests but i would see about how many links you can overlap on the sprocket and then i would just cut that many out of the chain. chain breakers are super cheap compared to a whole new chain.
Motorcycle chain breaker
But if your sent on a new chain i usually get mine at the same place they also have tons of sprockets that should fit better than a 40.
Motorcycle chain.