>>24668981 (OP)
Classical republic as in truly classical, or ((classical)) as in Enlightenment/Renasisance looking back? The two examples make me think the latter but if you mean the former then Constitution of Athens is the only one actually preserved from a Greek poleis.
The Napoleonic client state constitutions are obviously later than Rousseau et al, but you could strip some of the newer ideas out and be left with something interesting. Also, San Marino still operates under it's constitution from 1600 so that would be a fun way to start. One I never see mentioned is picrel, which being a colonial constitution is an interesting blend of super reactionary and cutting edge. Slavery is explicitly protected but lawyers are outlawed and every law passed by the legislature expires after a hundred years