The 650NK and 650TR aka 650YK share the same running gear (engine, transmission etc). If you are looking at just the muffler, 'end-can' that removes the catalytic converter or 'box of tricks' then look at some of the threads/posts listed in this CFMoto 650 Compilation thread as there are a few CF650's that have undergone that muffler transplant including my 650TR.
If you're looking at replacing the whole exhaust system (header pipes included) with something like a Yoshi exhaust system, then that is a different proposition and IMO you'll need a specialist tuner for that...

YMMV