Most NPCs use a flat-basic stat block in lieu of character classes. Please observe the Diviner, a 15th level caster, from Volo's Guide to Monsters. This Diviner wizard has (1) class feature, the daily use of one 'Portent' ability (similar to the Wizard class ability):
Portent (Recharges after the Diviner Casts a Divination Spell of 1st Level or Higher). When the diviner or a creature it can see makes an attack roll, a saving throw, or an ability check, the diviner can roll a d20 and choose to use this roll in place of the attack roll, saving throw, or ability check.
In contrast with the Divination Wizard sub-class, NPC wizards can 'recharge' this Portent-ability after the use of any spell from the Divination spell-list.
Explanation on relevance: PCs tend to lack Legendary Resistances and equivalents. Such a Diviner could modify any save or suck spell or situation. Say this wizard observes a PC group against a Beholder, Medusa or Dragon, they need only cast a single divination spell to change dice rolls accordingly. Less need for a Long Rest. Logically, these wizards (of 18+ intelligence) would want as many (cheap) divination spell slots as possible.
Thus, such a wizard would seek to restore spell slots via things like a Pearl of Power. As the magic item list in 5e is extensive, let us focus on spell scrolls. These magic items are relatively easy to make (1st level spell-scrolls are 'common' magic items) and seem to approximate a wizard casting their own spell.
If it is relevant, the answer below can include spell scrolls made specifically by this Diviner-wizard themselves &/or those made by other caster-enchanter-scribes. Does it make a difference if they are casting their 'own' spell or not?
If possible, I would be thankful if the answer could also include the infamous Rings or Ioun Stones of Spell Storing. Does it make a difference if the wizard is regaining their 'own' spell via some external source?
Question Specific & Detailed: Can the NPC 'Divination' wizard (from Volo's Guide) restore their daily Portent ability by casting a divination-type spell from a spell scroll magical item - or can this Portent-reuse only be gained via memory-slot spell casting?