minecraft-server-resource-pack/packs/0-base/assets/minecraft/models/item/iron_nugget.json

{
  "parent": "minecraft:item/generated",
  "textures": {
    "layer0": "minecraft:item/iron_nugget"
  },
  "overrides": [
    {"predicate": {"custom_model_data":1}, "model": "block/custom/chess-pawn"}
   ]
}