I am looking for a flying wing and found the Zeta FX-79. I have a brushless outrunner in the size 37-20 and 630Kv in my drawer. I also have a 3s-4s 50Amp ESC. I could use two to four batteries 4s 1800mAh 60C in this wing. Do the components fit for the FX-79?

Well that depends on what you want for top speed.
If you build for 2000 grams AUW and 4s that would put you in the efficiency band for only about 75kph top speed.

You could serialize your 3s into some 6s goodness for something more like 120kph top speed.

I can't use 6s with my ESC.
I found out that the FX-79 already comes with motor and ESC. Or is there a version without motor and ESC?