I raced an MC18 with MC16 motor for a while-till I could get some MC18 cylinders.
I had the 1989 MC18 carbs (open airbox), heads, full PGMII electrics and pipes on it.

Strangely, it went a lot faster...

Yes, if you get those carbs, you will have to rejet. On the bright side, the JHA MC21 jet-kit fits. It did on mine, at least.
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The Chopster