I would look for riding breeches patterns, and a basic jacket pattern and customise it with needed pockets (they look like they're just box pleats and a flap) and trimmings. You could even just use a mandarin-style jacket pattern and add the collar fall. From the picture, it looks like a hidden-button closure - but you could just as easily install a zipper in the flap, too.
You could even just let out the thighs on a high-waisted pants pattern - a lot I've seen just go straight down from the hips and then curve in at the knee, some have a slight added fullness in the inner-seam, too - but it's slight.
I don't really know anything about actual WWII uniforms - I'm just going off the reference and a quick Google search.