Sound's like a NX5 or NXA motor, - but the chain would now be on the wrong side - He must have the RS swingarm to go with it? Is this an ex race bike? I don't know what setup he might have re lights (total loss/separate battery etc). Crank tapers are different between RS & NSR' which is why more people aren't bolting RS engine bits into their roadbikes - and the later RS are much too different anyway.......

Not sure re the jetting, how much it could be de-tuned for road use, but cranks generally are rated for 2000km's race use or there abouts. There are guys on this forum that have RS race bikes, but this isn't the section for talking about HRC stuff, and this thread will probably get locked or moved.