This simple script uses tr:

tr -d '\r' < "${source_filepath}" > "${source_filepath}".txt;

Convert old Mac OS line endings to modern text files using dos2unix.

And here you go:

find -E . -type f |\
  while read source_filepath
  do
    dos2unix -k -n -c mac "${source_filepath}" "${source_filepath}".txt;
  done

Lowercase file names and change spaces to underscores.

find . -depth -name "* *" -type f |\
  while read source_filepath
  do
    destination_filepath=$(echo "${source_filepath}" | tr A-Z a-z | tr -s ' ' | tr ' ' '_'| sed 's/_\./\./');
    mv -f "${source_filepath}" "${destination_filepath}";
  done