If it sounds as though this does not include Zechariah, this phrase could be translated as "Many of you descendants of Israel" or "Many of you who are God's people, Israel." If this change is made, make sure that "their God" is also changed to "your (plural) God."
"with the same spirit and power that Elijah had." The word "spirit" either refers to God's Holy Spirit or to Elijah's attitude or way of thinking. Make sure that the word "spirit" does not mean ghost or evil spirit.
The heart is spoken of as if it is something that can be turned to go in a different direction. This refers to changing someone's attitude toward something. (See: [[rc://en/ta/man/translate/figs-metaphor]])
What the people will be prepared to do can be stated clearly. AT: "make ready for the Lord a people who are prepared to believe his message" (See: [[rc://en/ta/man/translate/figs-explicit]])