Have a big problem with the catalytic converter my mazda 6 2.0 2002 was stolen and I can't find a new one anywhere, but for 1.8 there are plenty of them Does anyone know what separates them, can I modify it to 1.8 so it fits my 2.0 or is it the cone with the manifold that differs, well really the only thing I can't modify

 

The CAT internally in itself does the same thing there is no difference in the Emission at the tail pipe.
If you can modify it in a way that looks more or less factory most countries would except it. But I would not recommend to use any none OEM clamps and welding would be a better installation.

 

Welcome to the forum

 

Yes, welding and pipes are no problem, but the biggest concern is that the cone separates, well that's the only thing I have difficulty modifying

 

I suggest to change your font color on some devices (like the one I am using at the moment I can barely read you response? ^^^^^^

Most exhaust that do separation are after the CAT so it may take a muffler shop to help you modify a OEM CAT. You are not bond to necessarily use only a MAZDA CAT as most are either 2 way or 3 way CAT.
YOur year I believe (didnt look) is a 2 way so either will work. If yu had a 2014 or newer then it would be a 3 way and and 2 way CAT wouild not work .