Make a flange from mild steel plate, it should be at least as thick as the stock exhaust flange, slightly thicker won't hurt and would make the flange more rigid, stronger. Don't go too thick, though, or you'll need longer studs. I've bought mild steel plate on eBay but there are other sources. There are DIY header kits that include 2 or 3 mandrel bends with legs (straight tubing) and a generic clone flange (that won't work for your MM, from what I've read) available on eBay and elsewhere. I bought one but never used it because I then found an NR Racing header that was just what I wanted. Individual mandrel bends in 1" are kinda' hard to find (plenty of larger dia. ones), but there are some sources. To fab a header, you wouldn't necessarily use entire bent sections; most likely you would cut pieces from them and fit them together to get what you want. You could get the most use per dollar from a 180 (you could get a 90 and a couple of 45s from it, for example). What I would do is fab the header on the bike (with proper precautions such as removing fuel tank, fuel line and carburetor). To (over) simplify it, say tack a tight radius 90 to the flange that points rearward, a short straight section (which could be a leg of the bend), then a 45 turning out to the side (to clear the frame), then another 45 to the rear, maybe adjust the two 45s to get a slight upward angle to match the rake of the luggage rack on the MM, then a straight section and muffler. Tight radius bends would, IMO, make fitting easier and result in a neater looking/less bulky header, I would especially want a tight bend at the flange to keep that first bend from sticking out too far, it will be the hottest part of the header. Easy to picture in my head, not so easy to explain. If you aren't equipped to tack as you go, I have an idea on how to work around that.
Here's a DIY kit, it has rather large radius bends; it's the only one that came up in an eBay search, surprisingly.
http://rover.ebay.com/rover/1/711-5...0001&campid=5335845805&icep_item=192306668897
http://www.mandrelbends.com/mandrel-bends/mild-steel-16-gauge.html
A tight radius 180
https://www.verociousmotorsports.co...Verocious-180-Degree-Mandrel-Bends-Mild-Steel