Before the sound restrictions all the big-V8 drag cars ran 'open headers'. You want the SLIGHT backpressure to keep the valves from burning, but you also want that spent gas the hell out of the engine. You don't want to run any engine without some pipes from the head. Having them 'tuned' is not for sound (although they typically sound better) but it's for improved scavenging of gasses.
Some tracks still allow open headers. It's typically been better than any exhaust, but the technology available seeems to indicate otherwise...
I had a '67 Plymouth Belvedere a few years ago. That was a monster. It had a 0.060" over-bored 440 (453 C.I.) and the largest cam I could install without notching the pistons (0.513" lift and 292 degree duration if memory serves) and about a Billion dollars in other mods... But I learned a great deal about pipe diameters of the primary tubes, exhaust pipe diameter, mufflers... (my primary tubes were 1 5/8 inch, collected to 3" head pipes running back to reducing cones into 2 1/2 inch Flowmasters and 2 1/2 inch tail pipes. It was not much quiter than the 3" head pipes with no mufflers... I had no H-pipe on this car, as I was not in need of any torque.) That particular motor was rev-happy. I shifted at 6500RPM (insanely high for a big-block) and it was still pulling HARD. I just did not have the balls to rev it any more. I had visions if push rods coming through the hood....
BUT, that was for an American V8 built LONG BEFORE the cat was invented...
For a high-revving (American V
engine, you want large diameter PRIMARY tubes and exhaust tubes, at the expense of low-end torque. An H-pipe is typically added to enhance the low end torque. This is popular on 5.0 Mustangs. Since the 1.9 has no torque, I would go with smaller primary tubes and exhaust, with a "try-Y" design header. ( [2 into 1 + 2 into 1] into 1- doest this make sence?) But I have no idea if one even exists. If you can find an EXCELLENT TIG welder, he (or she) may be able to make a 2 into 1 pipe that fits onto the stock header, essentially creating a 'try-y' header from the stock header.
Now, for the pipe, you will have to experiment with different diameters and a dyno to find the best size for the flattest (and highest) torque curve.
This is why exhausts cost so much. Not for the materials, but the R+D.