You could also change the article before business owing to the relative clause (that it was in the forties). In the old days the was usually used before radio but my understanding is that it is often omitted in contexts like your sentence nowadays.
Although it is no longer the big business that it was in the forties, [the] radio continues to be a medium of essential communication, especially at local level.
It many languages, including Finnish, the structure of the sentence would be unacceptable as the reader must wait for the middle of the sentence until he is told what it refers to. My personal preference would be to let speakers of English enjoy the same straightforwardness:
Although the radio is no longer the big business that it was in the forties, it continues to be a medium of essential communication, especially at local level.
CB