The beauty of the public domain, of course, is that there are no *rules* for sourcing, and you don't *need* to do any at all! But you're right that it's still considered good etiquette to source the work you use if it doesn't interfere with your own creativity to do so, it's just not strictly necessary. It's also a good idea for the purposes of the jam, so we can easily see that the entry does indeed make use of at least one 1927 work.

The creator name and work title is generally all you need - and you can include more information if you wish. You can include them in the actual rulebook or somewhere else in the game materials, or just put them in the game's description on its Itch page if you prefer!