Kemush is the idolatry of Mo'av (Bamidbar 21:29). Molech is Amon's idolatry (Melachim 1:11:7)!
Radak: You demand what used to be of Mo'av. Sichon took it from Mo'av, and their god did not save it. If it was like you say (it was your land), Kemush is your god.
R. Chaim Paltiel (Bamidbar 21:13): Mo'av conquered from Amon their land and their god (and later Sichon from Mo'av).
Ramban (Bamidbar 21:29): Kemosh was also one of the gods of Amon, like it says here.
Bechor Shor, Hadar Zekenim (Bamidbar 21:29): [Even though the verse begins "Oy Lecha Mo'av,] Am Kemosh" refers to Amon, like it says here. 1
Refer to 11:15:1:3.
Why did he say "what Kemush your god gives to you?"?
Ralbag: He was teasing him. Kemush could not save them from Sichon!
Emes l'Yakov (Devarim 4:19): The Ramban holds that each nation has an angel over it. What your god (angel) wants to give to you, you will have!
