Thanks for the replies.
I actually do still have all of the default fonts installed - if I go to the Software Center or attempt to install them via eopkg, it says they're already installed. (Apologies, should have said that to begin with).
I just can't seem to find them anymore.
If I try to remove them, it has me remove Budgie dependencies, which I didn't want to do.
Edit: Manually downloading the missing fonts, putting them in
~/.local/share/fonts/ and running
rc-cache -rv worked.
Re-edit: I am now getting
Fontconfig error: Cannot load default config file and the lock screen just has white boxes anywhere font should be.