I assume the last seven characters on a private key is based on checksum then the search would have been 3*58.
I tried it on FinderOuter but the search is 3*58*58*58*58*58*58*58*58.
Ie 3=first character after 5.
Than 58 for one invalid character.
Then last seven characters for checksum ( 58 x 7 ).
Had FinderOuter provisions for elimination of checksum and calculated only the main characters then the search for valid key would have been very simple.
1. Checksum part for WIF private key starting with '5' is
only last 5 character (Not 7)
2. If you assured that there are 2 Char wrong, So all possible valid key that can generated is only 3x58. ( = 174 key)
3. After convert those key to P2PKH address (BTC address starting with '1'),You can check all of that 174 address if it have balance or not. (It takes less than 1 minute in bitcoin blockchain inquiry API)
All the above processes can be done with one python script