Cheap and shonky window replacement best solution?? Query

Well it turns out that the aluminium window in the second bedroom was installed without a decent lintel.
Window is 1900 wide and the "Lintel" is an old "4B2" not even on edge and I can see a bow in the window.

We have decided we probably will not be going up a floor but just in case I want to frame up to cover this possibility.
thinking about things this morning and I wonder what is the best solution, to cut out all the shonky work and add new timber studs and jack studs and a proper lintel to suit a 1900 opening or to re-frame for 2 smaller windows and get two "off-the-shelf" awning windows to fill the holes>??
Second part of the question, is there any structural difference or ease of building between the two styles of lintel.
ie lintel under top-plate or lintel over the opening??

Dividing into 2 separate opening does allow the use of standard 90*35 as the lintel, but I happen to have on hand a length of 140*35 F22


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