I did some searching, and the quote originally in Greek, from M. Antonius Imperator Ad Se Ipsum, 10.16
Μηκέθ̓ ὅλως περὶ τοῦ οἷόν τινα εἶναι τὸν ἀγαθὸν ἄνδρα διαλέγεσθαι, ἀλλὰ εἶναι τοιοῦτον.
To match the Greek—particularly τοιοῦτον—you'd use talis, meaning "such", to mean "one". I think this would be the option for most cases like this one. As for "be", strictly speaking the singular imperative is "es", and the plural "este". This makes "be one" es talis (speaking to one person) or este talis (speaking to multiple people, where talis is still singular because it agrees with an implied singular subject.)
I think the Greek uses an infinitive construction, viable in Latin as well. It says something like "Don't decide anymore what makes a good man, but instead to be one". Keep in mind, I don't actually know Greek, but in that scenario "to be one" would be in Latin esse talem.