ἐπιμνησθεῖσιν πρέπον ἂν ἡμῖν εἴη σοί τε ἀποδοῦναι χάριν καὶ τὴν θεὸν ἅμα ἐν τῇ πανηγύρει δικαίως τε καὶ ἀληθῶς οἷόνπερ ὑμνοῦντας ἐγκωμιάζειν.
what is exactly this οἷόνπερ? ‘as’?
it wound befit us, having mentioning it, to pay you back and to chant the goddess?
οἷόνπερ is used much like ὡς or ὥσπερ, going with the participle. ἐγκωμιάζειν is in parallel with ἀποδοῦναι χάριν, and οἷόνπερ ὑμνοῦντας is quasi-parenthetical.