I've got an MC16 with a lot of MC18 bits. The MC18 tail seemed to line up perfectly with the mounting holes on the MC16 subframe, but there were issues with fitting the tail light, and the undertray interfering with the tail.

I ended up putting on an MC18 subframe, which is a perfect fit. I've tried out the front and upper fairings from an MC18. But they didn't quite work. As Andy said, the upper fairing will not fit the MC16 brace (and the MC18 one can't be easily fitted), and I think there were also clearance issues with the MC16 oil tank. I can't remember if the issue was with the upper or lower front fairing.

Note: the parts I've tried to fit are all from an r5k MC18, the the r2j may be a different story.