Shot in the dark
Well, this is just sort of a bit of a guess...
But I could swear I'd read before that the original X1/9 show car was using 850 running gear, or something like that.
No idea what engine or anything else that might have been shared...
But looking at the photos of this car at a few different angles, well, it looks to me more like they took an 850 spider, and sort of shaved off the top and/or dummied in a targa hoop and stuck a targa top on top of that.
The wheels look straight off of an 850, the tail lights look like the were stolen off of a 65-67 850 coupe.
The way the wheel wells go out at an angle towards the top and large front overhang also seen very 850 spider like to me, as well as the frogeye look of the nose...
Anyway, you asked about the muffler...
Well, if that really is something like 50% 850, then why not the muffler too?
The 850s had some different style mufflers over the years, but what that one reminded me of was what you might end up with if you took one of the ones that had the sort of snakey "S" shaped tailpipe, turned it sideways, and then cut tailpipe off a bit short.
They have a picture of the kind of muffler I was thinking of over at vickauto:
http://www.vickauto.com/newstore/index.php?main_page=product_info&products_id=2414
Or, if links to other sites are bad, an image search for:
Muffler, Fiat 850 - (SKU 27-8354)
should get it.
Anyway, like I said, that's just speculation on my part. I could be toooootally wrong