Try this :

 find . -type f -print0 | xargs -0 -r grep -Z -L -U '[^ -~]' | xargs -0 -r flip -u

Where the argument of grep '[^ -~]' is '[^<tab><space>-~]'.

If you type it on a shell command line, type <ctrl+v> before <tab>.
In an editor, there should be no problem.

- '[^<tab><space>-~]' will match any character which is not ascii text (carry reutrn are ignore by grep).
- -L will print only the filename of files who does not match
- -Z will output filenames separated with a null character (for xargs -0)