I'm gonna step way out on a limb here and try to help you. I remember back in the 70's, Chevrolet used air cleaners on their 350's that had a long snorkel tube on them. If I had to guess (and if you trust my memory
), the diameter of the opening was about 7cm. All the air taken in by the motor passed through that opening. To figure the area would be: 0.7854 X 7 X 7, or 38.48 square cm. Now, you'll have to realize that these motors were of the smog variety and were probably all done by 4000 r's, so if you were to figure a 350 at 5000, you'd multiply 38.48 X 1.25 and get 48.1 square cm. Figuring for 6000 r's would be 38.48 X 1.5 and you'd get 57.72 square cm.
Now, looking at the photo you posted, I would guesstimate the opening at 50 cm wide by 15 cm tall and would identify it as an elliptical segment of an ellipse having a width of 50 cm and a height of 30 cm. To find the area of HALF an ellipse, multiply the width times the height times 0.7854 times 0.5 So, 50 X 30 X .7854 X 0.5 = 589.05 square cm.
I know I'm flying by the seat of my pants, but maybe I've provided you something to work with.