I use Pixaki, alongside some old Machintosh apps in an emulator: https://blog.gingerbeardman.com/2021/04/17/turning-an-ipad-pro-into-the-ultimate-classic-macintosh/
You might also have some luck with the web apps listed in https://devforum.play.date/t/convert-color-pngs-to-1-bit-per-pixel/393