One of the members of a 850 forum tipped me off about a stock of slightly rusted mufflers that were for sale at Martin Willems. They were Abarth lookalikes that could make a nice sports muffler with not too much work. And because the budget is not infinite, I took the chance. For 50 euros (plus the shipping costs) it was deliverd nextdoor (because I was working). This is how I received it (in this case an exhaust for a Seat 133):
Fiat 850 Uitlaat

Tried with a brush and sanding pads, but blasting gets better results!

Fiat 850 Uitlaat

And then lots of layers of heat-resistant lacquer. There is still a plan to spray the ends with chrome lacquer, but for now I'm leaving it this way.

Fiat 850 Uitlaat

In our new-old stock there was also this manifold, originally intended for a 600. But the flange fits perfectly on the cylinder head of a Fiat 850..

Fiat 850 Uitlaat

So we have a new challenge: fixing the connection between the manifold and the silencer.