You can get an RG500 motor into an NSR chassis - its been done already but is a bit more work that most can deal with I hear.
100bhp at the wheel is relatively easy to get to as opposed to almost impossible with the NS400 motor, some bikes owned by 'enthusiastic' owners with a few quid spare each month see a reliable 110bhp before you start looking at big money to go further.
110bhp NSR, hmm, light, handles ok and looks good.
I bet the frame would die after a some serious though
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