Find Familiar says:
Finally, when you cast a spell with a range of touch, your familiar can deliver the spell as if it had cast the spell. Your familiar must be within 100 feet of you, and it must use its reaction to deliver the spell when you cast it. If the spell requires an attack roll, you use your attack modifier for the roll.
Emphasis mine.
And Magic Stone says:
You touch one to three pebbles and imbue them with magic. You or someone else can make a ranged spell attack with one of the pebbles by throwing it or hurling it with a sling.
I'm confident the familiar could touch the stones themselves, and deliver them to somebody else to use.
But does this spell "require" an attack?
If it does, then these both seem like exceptions to the general rule about Familiars not being able to attack. Otherwise can you read Magic Stone as 'granting' the spell attack, or does it not and only creatures that can attack can "make use of" the new spell attack option?
Either way can the familiar fling the stone and make an attack?
Best Answer
Magic stone changes the properties of a rock, not the properties of a creature.
The specific beats general rule states:
Magic stone states:
This is a general rule in the sense that it isn't creating any exceptions other than "this rock is more special than other rocks". Magic stone is about changing the properties of the rock, as it targets the rock, not changing the properties of creatures that hold the rock. To put it another way, "people who can throw rocks can throw the special rock in this special way", not "people who can't throw rocks at all can now throw rocks".
Find familiar states:
Since throwing a magic stone calls for making a spell attack, and a familiar cannot attack, a familiar cannot attack with a magic stone. Magic stone does not explicitly grant creatures that cannot attack with rocks a means of attacking with rocks, so a familiar cannot attack with a magic stone.
For an example of how the rules make exceptions like this, consider the Nature's Mantle from Tasha's Cauldron of Everything:
You might be tempted to say that the Nature's Mantle lets you hide as a bonus action even when you normally cannot, such as when being directly observed:
And you would be right, because I left off the rest of the magic item description:
As you can see, Nature's Mantle explicitly creates the exception to being able to do something you normally would not be able to. Magic stone makes no such exception for creatures that normally cannot attack.