If you make the header 660mm long from the valve then into a good muffler that should work nice at 10000 rpm if it makes its power in another rev range add 50mm to move power down the rev rang (ie 710mm header length = 9500) or shorten it to move it up the rev range (ie 610mm header length = 10500). Header dia I would leave as standard 4 now. Bigger might not be better for you yet. A good header length for an FXR is around the 500 550mm from valve as they make there power at higher revs.
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