Large cracks at front of house? HELP!

Hey guys so we are renovating a house we recently bought and there were a few hairline cracks in the render which have been pollyfilled but there is this one large crack that I'm stumped on!.

This crack is about 10-15mm thick and is horizontal. Today I got a better look and peeled up the loose render around it and noticed a slightly rusted arch bar but still structally safe.

I was wondering if I could put a rust kill on it then render back over the top? I am hoping that I do not need to replace the arch bar or lintal next to it .

Also before buying this house There was a huge palm tree removed 3m from the house foundations.....

If anyone has any advice or information on this topic it would mean the world to hear from you! Thankyou

renokid Nice art deco house reno
What condition is your roof and flashing?
At a guess I would say the parapet flashings & Gutters have failed with water ingress down your cavities,
resulting in your lintels rusting..unfortunately they will need replacing.. no easy fix

Nice one! But I think it wont be that easy to fix