This comes from Cavalcanti's:—
E cantinne gli augelli.
Ciascuno in suo latino.
Stanza 45 is taken bodily from Claudian, Dante, and Cavalcanti. It would seem as though Poliziano wished to show that the classic and medieval literature of Italy was all one, and that a poet of the Renaissance could carry on the continuous tradition in his own style. A, line in stanza 54 seems perfectly original:—
E già dall'alte ville il fumo esala.
It comes straight from Virgil:—
Et jam summa pocul villarum culmina fumant.
In the next stanza the line—
Tal che 'l ciel tutto rasserenò d'intorno,
is Petrarch's. So in the 56th, is the phrase 'il dolce andar celeste.' In stanza 57—