Idle surge after catalytic converter change

[MooN]

Hey all! I have an 87 325i (M20). I had it running amazing. Started up great too. Well, I couldn't pass smog so I had the catalytic converter changed. After this I am now having surges after the car is started, rough idle, bad starting. The cat swap required some changes to have it fit. Went from a 2 in 2 out, to a 1 in 2 out. The "Y" converter pipe is pre cat. Would this cause the issue? Any ideas?
Thanks!

[Panici]

Is the O2 sensor before or after the cat?

Did they disconnect the battery when welding up the exhaust?

[E30Bastard]

Did they even reinstall the O2 sensor at all haha