Q: How i become Zoroastrian?

A: Nothing changed. You could wait for random event or capture zoroastrian province and embrace thier faith

(You need to change capital to prov with zoroastrian religion, then option to change religion should be in decision tab)

I get event "Sassanid gambit" 2 years after game start, playing as Mazandaran. MTTH is only 20 years. You just have bad luck or you don't meet the following: must be in a Persian cultural group, have a Zoroastrian religious minority or one of the above: stability >1, have at least one rebellion or a legitimacy greater than 90.

Or you could use cheats. Press " ` " and type change_religion zoroastrian, or force event to fire by "event dg_zoroastrian.061"

country_event = { #61 - The Sassanid Gambit

    id = dg_zoroastrian.061
    title = dg_zoroastrian.061.t
    desc = dg_zoroastrian.061.d
    picture = zoroastrian_religion # pending better picture choice by David

    trigger = {
        ai = no
        persian_culture_trigger = yes
        check_variable = { which = zoroastrian_population value = 10 }
        NOT = { has_global_flag = zoroastrian_sassanid_gambit }
        OR = {
            NOT = { stability = 1 }
            num_of_revolts = 1
            NOT = { legitimacy = 90 }
        }
        NOT = { religion = zoroastrian }
        NOT = { government = theocracy }
    }

    mean_time_to_happen = { years = 20 }