I've been wondering if one could purchase some metric tubing nuts from an auto parts store or something? You could make a header out of the mild steel fuel tube, put a flare on the end, and you'd have a header as long as you need. All ya gotta do is bend it to whatever shape you need.
I did that on an old Saito .80, but I just used the header that came with the engine and brazed on an extension. I also brazed a pressure fitting in the tube that had a 45 degree angle on the end facing the oncomming exh. gasses. I got a little bit of tank pressure with that set-up, but not as much as with the stock muffler...