You can use morbus instead of sana if you put it as "mens morba in corpore morbo". Morbus is the concept of sick as in "diseased". You might consider using the adjective aeger instead, since it also means sick, but in the sense of "infirm" or "weak", and indeed one of its definitions in the dictionary is "unsound".
So, with these words you can have either one of these:
"Mens morba in corpore morbo."
"Mens aegra in corpore aegro."
Also, perhaps using insanus, the opposite of sanus:
"Mens insana in corpore insano."
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae