To —— 'Had I a man's fair form, then might my sighs
ivory shell
Keats uses the period's standard anatomical metaphor—the ear's spiral shape resembles a seashell. He's being formal, even clinical, about the body he can't touch.
No cuirass glistens
A cuirass is chest armor. Keats lists three masculine roles he can't claim: knight, warrior, shepherd-lover. This is a catalog of his social inadequacy.
Hybla's honied roses
Hybla was a Sicilian town famous in antiquity for honey (not roses). Keats conflates two separate symbols of sweetness into an impossible hybrid.
spells, and incantation
The poem pivots from self-pity to fantasy: if he can't win her as a man, he'll use magic. The shift from social failure to supernatural ambition is abrupt and slightly desperate.