The NSR uses a 520 chain. You want to get one of the lightest chains available.

Honestly the bike will waste more energy then needed spinning around a heavy duty chain meant for a bike like a 750 or bigger. THe powere produced by these things isn't insane, so get a normal chain (maybe not even o-ring). THe lightest weight per link as possible will be your best bet, and save you money over the high prices X-Ring, fancy chains meant for the latest GSXR or R6.
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Charles Gallant