I'm not sure even the collective brain power of the entire header industry could answer your question since most of them might want to know...... mind you, this is just a guess on my part - they might want to know what kind of motor is in your car.
On the basis that we here at hotrod central don't really care whether it's a Yugo, Corvair, or Allis Chalmers motor - our mission is to help.
Personally, I'd plan on building my own headers. I've done it before and because I have, let me be the first to suggest you not use the mediocre quality of tubing bends offered by everyone who's bends are made in Mexico. The metal quality is unacceptable.
I've (tried to) make headers for my drag bike only to discover I could not TIG these pieces of poo-poo-number-2-grunty-in-the-pants steel bends together due to their poor material quality. I had to gas weld the junk tubing together.
So, buy or make a flange or flanges for your mystery motor and make the headers. But, use stainless steel tubing like that sold by Burns Stainless in Costa Mesa. Pay someone to TIG'em if need be. By the way, polish the tubing before welding.