Hello, I had a similar problem, working with some .notebook files that I had to decrypt.
So I fiddled a bit around with a hexeditor and found a workaround: I replaced the first 512 bits of the damaged file with 512 bits from a file of the same file type, but a file that wasn't damaged.
Now this may cause a some other issues, but in my case it worked pretty well, so I wrote a program in java that takes all files from a certain folder and replace the first 512 bits with some bits that you specify.
If you want to try it out, here's a link to the source code:
Basically what you do is place all the damaged files in certain folder, and find the 512 bits of a working file (I used HxD for this).
When you copy it straight out of HxD it gives you a hex string with whitespace, my program doesn't work around this, so perhaps use something like this site first:
If you have any issues, please don't hesitate to write.
Only do this on backed up files, since I cannot guarantee that this works.