I, unfortunately, forgot maariv last night but I led the prayers for Shachris. Does my Chazara count as having repeated Shachris?

Shalom, Thank you for your question. Yes, a chazan who forgot to pray the evening service, may use the repetition of the shacharit Amidah as a “tashulim” make-up prayer for the forgotten ma’ariv. (See Shulchan Aruch, Orach Haim, 126, 4, Mishna Brurah on 108, comment 4). The chazen should have intent that the repetition is to be a make-up prayer for himself, and to be also a regular repetition for the congregation who need to hear the Amidah. Blessings.