If I Mystic Reversal a Drag Under and there's only 1 unit at a battlefield and another at base, do I have to reselect the battlefield unit or can I just have it fizzle?

When you use Mystic Reversal to take control of an opponent's Drag Under, you must make new choices for the spell as if you were playing it yourself. Because Drag Under requires a target, you are required to choose a legal target if one exists. You cannot choose to have the spell "fizzle" (fail to resolve) if a valid target is available on the board.

The Ruling

• Targeting Requirement: Drag Under targets a unit at a battlefield.
• Your Obligation: When you take control of the spell with Mystic Reversal, you must select a legal target for it. If there is a unit at a battlefield, you must choose it.
• Fizzling: You cannot choose to "fizzle" the spell by selecting no target if a valid target exists. A unit at your base is not a valid target for Drag Under because it is not at a battlefield.

Why this is the case

When you take control of a spell, you are effectively re-playing it, and you must satisfy its targeting requirements. If a valid target exists on the board (a unit at a battlefield), you are forced to select one.

If, after you take control of the spell, the only unit at a battlefield were to be moved to a base or removed from the board before Drag Under resolves, the spell would then "mistarget." In that specific scenario, the instruction to kill the unit would be ignored because it no longer has a valid target, and the spell would resolve without effect. However, you cannot intentionally create this state by choosing "no target" when you are initially making your selection.

Note: I could not fully verify this ruling against the retrieved rules.