It is true, 99% WWII planes dont have inflight radar, but they had radios and were informed by land based controllers and by spotter planes. All this information creates a more or less precise picture, but never in real time with dynamic refreshing.
A god idea may be to force the pilot to solicite that info. So, you click on a sector, this sector is marked as selected (highlighted) and then you click on "REPORT" button. In 15 seconds you'll have the static "picture" of that sector. Once you zoom in or out, the zoom is done from the center of the selected sector unless you select the center as your own plane (by a check). The static picture has static red dots indicating contacts (detected by friendly radar of into the visual range of friendy units), but if enough zoom is applided, the dots switch to icons indicating "small plane", "big plane", "CV", "GV", each icon followed by a bearing arrow. With the static picture comes also the sector enemy bars, based on the number of contacts reported into the picture. This way we'll have enemy bars only based on contacts into visual range of friendly forces or detected by radar. Any friendly base is considered as friendly force to check if a con is into its visual range (even having its radar disabled).
If a picture is being processed (into the 15 secs since solicited), no more pictures of that sector can be ordered.