@froggy is correct (and cites the appropriate passage) – even if a given mod is seemingly abandoned, if the original creator does not grant permission (and lack of response = doesn’t grant permission) then you cannot update/modify/whatever the mod.
You can, as @coasterspaul observes, release a mod with permission to modify without contacting you. But few, if any, of existing Stonehearth mods have such permissions attached.
And, as @froggy also wisely observes, while mods themselves should not be messed with without creator permssion, the idea behind a mod isn’t protected. So you’re free to create a mod which bears a resemblance to others… you’ll just have to create it from the ground up.