I own a 1989 MC18 R6k.
If I had a set of MC21 carbs lying around, I would make sure that they were jetted very close to standard and chuck them on the bike and go for a spin just out of pure curiosity.
I do not know the difference between the J81E and 2QZ needles, but I cannot imagine the difference is going to cause major problems. (I could be wrong).
That's just me, personally, and I would be pretty paranoid about things potentially being too lean.

I think you should be fine.
But that is nearly the same as saying, things could possibly NOT be fine.
Sorry if this post is not helpful.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...