Yes, The mc-21 wheel will fit on the 18 without any problems. The only thing to be aware of is the back of the bike will sit lower in the rear. There are a couple of fixes for that.
(1) You can flip the suspension linkage plates around. Not a big diffrence but will make the rear a little stiffer.
(2)I think in the suspension section there is a diagram to make new linkage plates that will bring the rear back up.
(3) Get a longer rear shock (this is what i did) Ohlins CBR600 F3 with ride height adjustment.
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