The answer is after all just RTFM. The solution is:
Unmounted filesystem can be fsck’ed like this.
losetup -e AES128 /dev/loop0 /path/to/cryptedfile
fsck -t ext2 -f -y /dev/loop0
losetup -d /dev/loop0
I found this in the loop-aes Readme
salvaged from Wayback Machine at 2025-12-05, original timestamp 2005-11-23