I tend to think we should write new hymns instead of 'fixing' old ones. Earth as female tends to get tossed out when doing inclusive language & that's a very powerful image in this hymn that I really like. Here are my suggestions. I'm not crazy about O soul, I was thinking of "Turn back, sinner," but it doesn't really feel right. My parenthetical suggestions use the technique I've seen used elsewhere of turning 'man' to 'us.'

Turn back, O soul, forswear thy foolish ways.
Old now is earth, and none may count her days.
Yet thou, her child, whose head is crowned with flame,
Still wilt not hear thine inner God proclaim,
"Turn back, O soul, forswear thy foolish ways."

Earth might be fair, and all seem glad and wise.
Age after age their tragic empires rise,
Built while they dream, and in that dreaming weep:
Would all (we) but wake from out their (our) haunted sleep,
Earth might be fair and all seem glad and wise.

Earth shall be fair, and all her people one:
Nor till that hour shall God's whole will be done.
Now, even now, once more from earth to sky,
Peals forth in joy the (our) old undaunted cry—
"Earth shall be fair, and all her folk be one!"