I have a stock header and an Arrow. The Arrow is I stalled so I cant get an ID for it. Od is very close to 41mm. This expands, just slightly, once it gets to the collector. At that point, it's closer to 45.
Very difficult to measure with the plastic in place. I have a set of 2012 stock headers. The only thing that stands out is the measurement right where it leaves the head. The ID would be larger, but there is a massive weld there. Its goes all the way around and measures 28mm. If I used the stock header, I think I would use my dremel tool and remove some material from that weld.
I remember doing something like that in my 2 stroke yz 250. I know it's not really related to this bike, but there were some nice gains to be had when those pieces were matched up,