If I attach Shady Spectacles to a non-token unit, such as Daring Poro, and have it become a copy of Ekko, Recurrent, will Deathknell trigger and resolves when that unit dies?

Yes — it triggers and resolves: you recycle the Daring Poro (currently a copy of Ekko) and ready your runes.

While Shady Spectacles is attached, Daring Poro has Ekko's rules text — including Deathknell — as copyable traits (Rule 477.1.b). When it's killed, the Deathknell trigger is added to the Chain as a pending item before it moves to the trash (Rule 808.1.d.2), and the trigger stays because the kill was not replaced (808.1.d.1).

The only reason a Reflection token copy of Ekko fails is that tokens cease to exist in the trash, so the mandatory "recycle me" cost can't be paid (FAQ #9947, Rule 183.1). A real card like Daring Poro stays in the trash, so you can pay the recycle cost (Rules 416.1.a, 416.3) and the effect readies your runes normally.