Nonbasic lands that are turned into Mountains are still nonbasic. Magus of the Moon does nothing to change the land's supertype.
Lands that are turned into Mountains will have ": Add to your mana pool.
If a nonbasic land has an ability that causes it to enter the battlefield tapped, it lose will that ability before it applies. The same is also true of any other abilities that modify how a land enters the battlefield or apply "as" a land enters the battlefield, such as the first ability of Cavern of Souls. This is a change from previous rulings.
Standard: Ikke lovlig
EDH / Commander: Restricted
Pauper: Ikke lovlig
Brawl: Ikke lovlig
Premodern: Ikke lovlig
93/94: Ikke lovlig
Magus of the Moon er Lovlig i disse blockene: Time Spiral