This would not be spoken like this in English. We would say, "This is not an old car but a new one." I know its not a direct translation, but still...
The suggested translation given above, "This is not a new car, but an old one," is correct and sounds natural to me in English. Yours is identical in structure, but has old and new reversed.
If the desired translation is not simply old, but "old one" then shouldn't it be "regit"?
No, the accusative form régit wouldn't be appropriate. It isn't the direct object of a transitive verb, which is when you need that accusative -t ending.
..., but it is old. (Why was it not accepted?)
The Hungarian translation that appeared for me had the numeral 1, not the word.
It is not accepting the suggested translation: "This is not an old car, but new."
Because it's the reverse: "This is not a new car, but old"