how do I replace an unknown brand casement window?
If your window is in good shape except for the fogginess of the glass, the insulated glass unit (IGU) can be changed out by a glass installer. They will make a house visit, or you can remove the sash and they will replace the IGU in their shop.

There are shops that will "fix" the old unit by drilling a hole in the glass and evacuating the moisture out of the cavity and leave a "breather hole" in the glass. I am sure this works, and a cheap fix too, but I would not hold my breath on this type of repair.