The FPP 650 pipe.
You can use a SJ "B" Pipe chamber but you will have to heat and melt the gas tank some to get clearance.
A FPP Wave Blaster Limited pipe will fit too but I think it will need a little welding on it to make it fit right.
That pipe on the yellow Aussie X2 was a rare FPP that is impossible to find so count it out.
I am going to try a WB FPP Limited chamber when I get around to it...
X2 Guy had a FPP 650 chamber for sale, 100 bucks + shipping. You can then just buy the head pipe from FPP. Would be cheaper and easier then trying to get FPP to sell you a new 650 pipe setup for $400.
Good luck. Search the forum, there are several threads about FPP pipes on X2's.
DBR