RS250's have aluminum thin walled tanks. Not sure if they would fit in an NSR frame without modifications to the frame.

The RS fuel tank kind of sits in the frame/airbox a little, so you may find the cross bar on the NSR frame will get in the way. Not 100% sure as I have never tried to match those two together. Also, I am talking about a 2001 RS tank, not sure of earlier model tanks.
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Charles Gallant