Header should be fine. I have a yoshi oval that I cut the dent off, riveted it back together, and it sounds decent. I have no idea about the rearsets.
Also, I've had quite a few different brands of 900rr headers and they can have goofy/ unique outlets from brand to brand. Nothing you can't get around with simple ingenuity and a welder though!
What kinds of parts are you thinking you might want?
30,000 mile 919 survivor. No plans of stopping the abuse any time soon.