Probably not very...
The exhausts definitely won't be - the pipe bends in different spots and where they come out is completely different. The Cooper has a single side pipe and the Cooper S has twin pipes that exit in the center.
Intakes may be interchangeable depending on the intake - the air box it would replace is the same between the two.
There are tuning modules, but I believe they are all made to work with the Cooper S engine and not the 3-cylinder.
The exhausts definitely won't be - the pipe bends in different spots and where they come out is completely different. The Cooper has a single side pipe and the Cooper S has twin pipes that exit in the center.
Intakes may be interchangeable depending on the intake - the air box it would replace is the same between the two.
There are tuning modules, but I believe they are all made to work with the Cooper S engine and not the 3-cylinder.