Measure from top of tooth to top of tooth and see what pitch the chain is. We'll try to be as simple as possible and just rely on pitch being that in your circumstance you'll have common chains most likely being what goes on the sprocket, examples being if you find 3/8" from top to top you'll end up with #35 chain or if you have 1/2" from top to top you'll have one of the 40's, ie: 420, 40, 41.