I'm using Krastorio 2, I got the Mk3 power armor, but when I equip it, the sprite reverts to the appearance of having no armor.
I tested this with the vanilla sprite and the engineer had the appearance of Power Armor.

Maybe this is an issue with some code? I haven't looked/found the vanilla sprites to see what could be causing it.

Hello. This mod only adds the armor sprites for default vanilla armor, so modded armor wouldn't be considered by the mod since I'm not the author of those mods.

This mod only adds the armor sprites for default vanilla armor,

That's OK. Custom graphics for custom armors would be nice, but are hard to implement.

so modded armor wouldn't be considered by the mod since I'm not the author of those mods.

But you could easily map custom armors to the existing animations! As a quick hack, just for "Krastorio2", I'd put this in data-final-fixes.lua (about line 10, right after you define CHAR_NAME):

-- Fix K2-armors 
if mods["Krastorio2"] then
  local char = data.raw.character[CHAR_NAME]
  table.insert(char.animations[3].armors, "power-armor-mk3")
  table.insert(char.animations[3].armors, "power-armor-mk4")

  local armors = data.raw["character-corpse"][CHAR_NAME.."-corpse"].armor_picture_mapping
  armors["power-armor-mk3"] = armors["power-armor-mk2"]
  armors["power-armor-mk4"] = armors["power-armor-mk2"]
end

By the way, I'd like to send you an update for the character generation/overwriting code one of these days. I've finally updated my character selector mode some time ago and improved the logic of my own character mod in the process. I think it would be a good idea to add this to Gear Girl as well. But it will still take me a while -- real life is interfering again. :-)