That is not a script, it is an old helper class to be used in different places within the program (from one of my old projects). It doesn't have any option to accept any "private" keys.
If you want to know how things are done you have to check the link I previously posted to the project called the FinderOuter.
For quick reference the entry point is here where "key" is the base58 encoded private key string containing missing characters that are replaced by a character defined by the missingChar char.
Initialize() method converts what it can from the given string to base-256 and sets the missing indexes. Then depending on key type (compressed or uncompressed) a LoopComp() or LoopUncomp() is called where different base-58 characters are placed in missing indexes and they are checked against the checksum. The rest are SHA-256 details.