Jason,

I have used the petcock from an early model CBR900RR on an NSR in the past.

The reserve tube and filter is a little longer, but that's fixed by using the reserver tube and filter from the NSR petcock.

Also, the black plastic turn knob is longer, but you can still use the NSR's for that. So basically your just using the petcock body. As for the NC30 and NS125? Not sure if they are the same. May have to swap some stuff like I did with the CBR900RR petcock if they are not "exactly" the same.
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Charles Gallant