Is there a tool that will recreate the fields in the back-end from the json file?
If the json file was created by ACF you should be able to just import it.
Yes of course, I meant from the PHP that can be exported..
to get the PHP back into JSON, you can do
$group = array(
// the PHP field group
);
$json = json_encode($group);
and then you can write the json to a file using PHP http://php.net/manual/en/function.file-put-contents.php. Once that’s done you can download the file and then import it.