I would just say "knock wood" (or "knock on wood"). Or, if you are translating within a Jewish context, you could just transliterate and say "keyn aynhora." (The way I'm used to hearing it, the pronunciation is more like "kin-anorra")
Here's what Wikipedia says:
Ashkenazi Jews in Europe and the Americas routinely exclaim Keyn aynhoreh! (also spelled Kein ayin hara!), meaning "No evil eye!" in Yiddish, to ward off a jinx after something or someone has been rashly praised or good news has been spoken aloud.
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I would use "God willing" in a different context than "knock wood."
What a beautiful baby - "knock on wood" (or "keyn aynhora"!)
The doctor says I can leave the hospital tomorrow -- God willing!