Similar to how you changed the icon, you can write to character.animations . However according to the docs it takes an array of animations https://lua-api.factorio.com/latest/prototypes/CharacterPrototype.html#animations.
So assuming animations is in a valid CharacterArmorAnimation format, you can ...

Hi!
I really need outside help to create a simple mod that would replace the sprite of the engineer. There are no problems with sprites (and their parameters like "shift_by_pixel" e.t.c.) and they are almost ready. The problem lies in the replacement itself, i.e. in the code.
What I want:
- This is ...