AUDI TT MK1 1.8T 150 AUM - Wiring Issues

Hey guys,

I have an AUDI TT MK1 1.8T 150hp AUM engine.

My car previously had AJQ engine 180hp(before I bought it). They swapped it for AUM engine. So the thing is;

Engine is AUM with AUM ECU 06A 906 032 DR BOSCH ME 7.5, but I think the Wiring harness is left for AJQ since there are no Relays(100,428 - no box either), and no Wideband o2 Sensor.
It pops PCM Relay Error fault code and S1B1 o2.
I was wondering if I replace the Big Plug Harness from AUM, will everything be okay as it should and no problems detected?

I want to add the Wideband o2 and the relays. The relays are somehow now bypassed? Car runs, no issues at all beside that it doesnt run as it should since its without an o2. Theres no hole for it too. So I have to weld and stuff.....So after explaining my issue can you tell me what would be the best option for me?

1. Replacing the complete loom + adding the relays + the o2.
2. Replacing the Big plug harness + adding the o2 + relays.
3. Buying this kit from unitronics? https://www.getunitronic.com/parts/accessories/uni-wideband-conversion-harnessAnd what's the process for this?
4. To wire the o2 from ecu? Mechanics and electricians arent really at their best here in Macedonia so I am so afraid for the last option lol.

THANK YOU!!