"The airplane has not been found" should be acceptable as well.
Doesn't "itu" mean "that?"
Yes. But itu has a lot of other meanings when being translated to English. "It" also means itu, and sometimes "the" can also translates as itu, and is put after the specified noun.
Based on a true story
"The airplane was not found yet" Would this be acceptable to?
From just looking at the Indonesian sentence, yes it should be acceptable. But it depends on the context, if the airplane is a specific one then "that airplane" should be used.