You don't need to bruteforce anything here (at least almost), and can recover the missing last 7 characters on an average PC at home within less than 1 second using a simple Python script!
In your picture there are 7 characters missing at the end of the private key (figured out by comparing with a test-printout using the same HTML file).
It has to be brute forced. There are a couple of ways but basically it is replacing each character by base-58 chars and checking the key.
The amount of time that process takes depends on the number of characters missing and slightly on whether the key is compressed or uncompressed and a little on your luck.
However if your key is compressed and is missing up ...