OPK files are read only, wich means they themselves don't contain config files. Traditionally GCW Zero programs store config files in (relative to FTP) /home/.program_name/ directory, which in case of mame4all could be something like /home/.mame4all, but you'd need to check. Directories starting with a 'dot' are by default hidden, so if you can't see them, you might want to turn on visibility of hidden files in your FTP program.