If you’re strictly looking for feature upvotes, Fider might be a better platform for your needs: https://fider.io/ It’s a bit like an open-source version of upvoty.
Otherwise, if you’re keen on staying with Publii but want to make use of an external component, I’d recommend:
Waline: It’s a self-hosted, open source comment system, but allows for customizable post reactions: https://waline.js.org/en/guide/features/reaction.html#customize-reactions
Applause button: Not quite upvotes, and hasn’t been updated in a while, but might be useful: https://github.com/ColinEberhardt/applause-button