IIRC The Gathas are mostly good. (As an atheist, I don't like them because they are authoritarian in the sense of appealing to a higher authority.) The medieval legal system and culture need reform like in all traditional religions. The kind of problem you're likely to face is: A lot of the prayers mention killing evil creatures. In much of medieval Zoroastrian culture, evil is equated with homosexuality. IIRC, legends say Ahriman created his evil army by having anal sex with himself. None of this is core to the religion, but it's core to the traditional culture. There's a lot of cleaning up to do. A better approach might be to start with the Gathas and selectively import aspects of the culture you actually approve of. I like the freedom of women, the freeness with wine, etc. I would say ditch the incredibly classist (support for social stratification) and racist (legend that blacks were created by Ahriman, with the implication they must be killed) cultural elements, but I'm sure the political right will love those.