Hi there.

I don't think there is a 42 tooth rear sprocket for the MC21 or it is
just not very common. It is very easy to convert the MC18's 42 tooth
to fit the 21. I made it fit on my bike and it is working well. You just need
to make the holes for the sprocket carrier bigger. And then just need to
press those 5 little hexagon spacers that goes into the rubber mounts
into the new sprocket. The rest is easy.