You could try K&S and put together your own. They have a multitude of exhaust flanges at this link:

http://www.krumscheid-metallwaren.de/html/fittings.html

They list them by exhaust opening dimensions (width and height), and bolt separation. I would've tried to look one up for you, but you didn't provide enough info (like the height of the opening), and "approx" doesn't cut it usually.

DB

warning - got to be good with brazing or silver soldering steel, or I would recommend getting someone to do this for you.