Good luck...hope it passes.
The problem is that even a brand new car a few months out of the showroom won't usually pass the ADR's. Motorbikes have had this issue for decades. For noise for example, they set a decibel limit and that's that...no leeway for wear and tear is usually allowed, and I got a "warning" about the stock factory exhaust on my 2008 Suzuki just after I bought it when it was only two years old. "Just over the dB limit there mate...but I'll let it go this time". This on a bike that had less than 10,000km on it, is absolutely unmodified, and which was literally like new.
Of course, Harleys are exempt from these testings under law apparently...that's how they can ride around with open pipes and literally zero muffling and never get booked...
Cars are a whole different kettle of fish. The problem is that once you change a muffler...even for a standard replacement aftermarket one...it probably won't meet the sound requirements that it was tested with at the factory.