Exhaust

It is probably the EGR valve that is open. You might have to make something to integrate the one that is one, was on, the other side as I believe there should be a matching valve on the opposite pipe that was there.
I would call the manufacturer of the exhaust and see if they make something for it. They should have adressed the issue of the valve in the instructions.
If you find in the end that the exhaust is un-satisfactory, as a high end aftermarket exhaust for a BMW should not have a loose end like that, you should get another one and sned it back. It is a sign of poor quality and maybe poor performance. Exhausts should last a lifetime so it wouls d be worth it. Try going to www. Carid. Com as they specialize in high end afterarket parts even covering parts for such cars as Bentleys and Lamborghinis.