Done correctly, ie, to keep the geometry of the mc21/28 the same, there is no way that you can simply swap the front ends over and find suitable bearings or at the least do a stem swap... If you don't care about your rake/trail and fork offset, then yeah, it's a piece of piss!

For reference, fork length from top yoke face to wheel spindle centre is 710mm, fork offset is 35mm. This will give you the 'standard' quoted rake and trail of 23 degress and 87mm respectively, although accurately measured it's slightly different, as it's also dependent on tyres and rear shock. Best thing to do is measure the 05 RR yokes, the forks will be way longer than the NSR's so that's not a problem.
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NSR300R - Why did i ever have a 250...