There is no per-unit stat for mana regen. There's a global mana regen per race that can be modified (but I don't know if it can be modified by an item), and there's a per-unit stat for tactical combat mana regen. You should be able to make an item that increases tactical mana regen, but you can't increase a single unit's mana regen on the strategic map.
There is a mana regen stat in the CoreUnitStats.xml, but it's commented out and not hooked up in the engine. Even if you uncomment it, it just doesn't do anything (according to earlier posts on the matter).
