Could you update this for 2.1?

No logic updates seem to be required. You can just extract the mod, swap some values in info.json, and run it just fine.

{
    "name": "kr-air-purifier-helper",
    "title": "Krastorio 2 - air purifier helper",
    "description": "Restocks air purifiers using construction robots.",
    "version": "2.1.0",
    "author": "Quezler",
    "factorio_version": "2.1",
    "dependencies": [
        "? newsletter-for-mods-made-by-quezler",

        "base >= 2.1.0",

        "Krastorio2Assets",

        "? Krastorio2",
        "? Krastorio2-spaced-out",
        "? k2-air-purifier",
        "? KrastorioAirPurifier",

        "(?) platform-construction-only-no-construction-robots"
    ]
}