Is This Basement Window Frame Structural
No standard 50s wooden window frame is supposed to be structural. The rough frame of a window can be, but you don't have one. 

It doesn't matter. You have a situation that requires a metal lintel. Brace the floor up just to be safe (I wouldn't work under those joists without something), install a lintel and then the window. Avoid lowering the bottom sill anywhere close to below grade if possible.

If I have to explain how to brace a floor up, install a lintel, or whether or not you need to do any of those things, it might be best left up to a professional.