No lintel above existing window for internal wall (double brick). Solutions?

Good afternoon,
[I have searched without answer - but if I have missed a relevant thread, please feel welcome to direct me]

Many thanks in advance for any assistance you are willing to offer.

I have attached a series of pictures to help explain the situation.

Painting the scene:
We recently bought a double brick house (1963) in Adelaide. There are no observable cracks anywhere, except for above the windows to the bedrooms (which appear to be lacking lintels in some places).

At this stage we are replacing the existing aluminium windows in the first bedroom. This window spans 2010mm to a height of 1350mm. There is a lintel observable spanning for the external brickwork, but I have been unable to locate a lintel for the internal wall. There appears to be some old cracking which has been previously filled on the internal wall. We wish to make good any structural deficits now so as to not have to deal with these later.

I have spoken to a few people about this. One chap said that there might be a concrete lintel previously poured into the cavity. Inspection from the roof, looking down from above shows this is not the case.

I have spoken to a builder who recommended use of the helifix helibar at intervals of 3 courses above the window.

An engineer I know has suggested that whilst an angle iron lintel would be optimal, a flat bar lintel may be easier to install.

My queries at this stage are:
1. What does the internal wall likely support in terms of load from above. Therefore, what structural support is required for this window.

2. Is there a form of lintel/support that I have missed? I have heard of a method of supporting the first and second course of bricks within the mortar work during construction. I am in the dark as to what this involves and whether it may have been used in this case. And whether there is any way to determine if it has been used.

3. If there is no lintel, what lintel is required? I guess from my perspective the goal is to stop future cracking. But, also I probably need to consider what rules need to be followed. I certainly don

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