Sync is a variation on Cipher. Is Sync to complex by making making the ability activated rather than triggered? I think by making a variant where the spell becomes an activated ability as opposed to a triggered one should lead a different play style than cipher.

I feel like the reminder text could be a lot clearer.
(You may pay an additional as you cast this spell. If you do, exile it encoded onto target creature you control. That creature gains ", : You may copy the encoded card. You may cast that copy without paying its mana cost.")

I want spell's effect to show up as an activated ability and not something cast. The first line does help clear things up. Is there a way to have numbers appear in the reminder text or must I do that manually?

The Xs in my comment will have to match the Sync cost; they were just an example.
If I understand correctly, you want creatures to gain the text of Sync spells as activated abilities. That's a very popular idea among custom card makers. It's also rather difficult to word.

Sync {N} (You may cast this spell for its sync cost. If you do, exile it encoded onto a creature you control as it resolves. That creature gains this spell's text as an activated ability with "{N}, :" as its cost.)

That's an attempt, but there may be better. Honestly, I wouldn't even try. Having the creature cast the spell is easier and more balanced.