The backup file is exactly identical, I'm afraid. Upon investigating the files, the corrupted content doesn't look much like a result of a software malfunction. Instead, it rather appears as if an entire part of the file were overwritten. Because the irregularities begin somewhere around the 4096-byte mark (typical cluster size on Windows), I'm suspecting it may be indicative of a more serious problem than one corrupted JJ2 level. For the sake of peace of mind, consider running CHKDSK to test integrity of the disk containing the J2L file.

Yes, the only damaged part of the file appears to be tile placement. If it's of any use to you, it would be possible to restore all other information, such as level and layer properties, as well as event placement.