It depends on the rough opening you have to work with as to what kind of window you can replace it with.
Do you have aluminum awning style windows or wood?
Aluminum awning windows often have a nailing flange that would be coverd by the outside casing.
Wooden awning windows, often use the outside casing as the nailing flange.
It Depends on what part of the world you live in, as to what product I would use. (and your taste)
I would prefer either vinyl clad or all vinyl windows. I'm not a big fan of aluminum clad windows.
I hope this helps.
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