Get the rear wheel off the NSR MC21, and a front of a VFR400 NC30 (or better yet, RS250 GP bike). That will give you a 4.5x17 rear, and a 3.5x17 front, to run 120 wide fronts and 150-160 wide rears...Fairly standard slick sizes.
Better yet... get 2 MC21 rears, have one set with wets, one set with slicks/Rennsports/pilot race etc...

Although Bridgestone still make 18inch slicks and wets
www.racinglinks.co.nz to find some...
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The Chopster