Metal basement window frame casement rotted out
I think I'd get some thin (1/4" or 3/8") vinyl plank and some white soffit nails and cover it as-is. Fill the outer section out flush with the inner stop using treated lumber and wrap it all. Caulk the joints blind as you assemble the parts. 

Here's a plan view section of the right side:

___________________ ___
   vinyl window    |   |
___________________|   |
                *|_    |&lt; -- old steel window frame
                *|x|   |
                *|x|   |
                *|_|___|*
                *********
                  ^--------- filler block (x)
                ^ ---------- vinyl plank wrap (*)