#GNU sed, 65

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    :                        # Define unnamed lable
    y/aiueo/iueoa/           # Rotate vowels
    /a/y/kstnhmyr/stnhmyrw/  # If at end of row, rotate consonants
    s/^a/ka/                 # If at end of vowel row, prefix a "k"
    /[yw][ie]\|wu/b          # If one of the invalid values, jump back to label and redo