Prior to the introduction of catalytic converters, mild steel exhaust systems could have a short life, especially on vehicles that travelled shorter distances. Combustion creates water, which sits in a cool exhaust as a corrosive mixture.
Catalytic converters get hot quite quickly, so the whole exhaust heats up more quickly, and most of the water will be emitted as steam. This allows the exhaust system to last significantly longer. Mine was well over 10 years old when replacement was needed.
As a result of this, the market for replacement exhausts is now small, where once it was huge. Many of the aftermarket manufacturers have disappeared. The cost of creating bespoke systems, with all the right bends, silencer boxes, mountings, etc., is only recovered with the highest demand applications.
1.4 500s are not plentiful, so if the system is significantly different to the 1.2, demand will be small, so aftermarket systems will be either expensive, or cheap and poorly made. A garage at the bottom of my road now rarely bothers with aftermarket systems, as they rarely fit properly, and need heating and reshaping. With that extra labour, an OE system becomes cheaper.
Even an OE replacement could be lower quality than the original. When the vehicle is in production, the manufacturer buys large batches of exhausts, so the exhaust manufacturer has a jig and production line, and uses quality steel. Once production ends, demand drops to a very low level. The original exhaust manufacturer changes to the new model. The specifications, and sometimes the tooling may then be moved to another manufacturer, better set up for smaller batch quantities. This happens with radiators too. Whilst made by a different manufacturer, they are still OE replacements as original specs are used.
Looking at
ePer, the parts listing, I see the centre section has a silencer, that is missing (not required) on the 1.2, but presumably due to different tailpipe designs, there are 8 different part numbers for the rear silencer. That makes sales volumes even smaller for each part.
From that, I'd not expect to find an aftermarket system at all, unless a manufacturer decides on the most popular, or perhaps the most aesthetic, and offers that to fit all. I think you have a simple choice of OE, or bespoke stainless.