If not, GitHub recently updated their search feature and it's pretty good.
https://github.com/search/advanced
https://docs.github.com/en/search-github/github-code-search/...
I think they made it harder to find - I remember being able to get to it in just the search bar. Having the syntax autocompleted for you would be nice.
The commonly used label is "help wanted".
You can dig further. Some ideas:
* Issues which are not to old.
* Issues in repos with an average amount of stars (~100).
* Issues with a limited amount of comments.
* Issues of users which project/coding style you like.
...
https://github.com/search/advanced
You can access all of this functionality with ghrabber.
One of my suggestions to Github is that they disable indexing of dotfiles of all persuasions (including contents of dot-directories), unless the repo owner explicitly opts in. That would make it much harder to find a very large fraction of the more obvious leaks.
You will need to enter the repositories by hand, though. Mind you, writing a small tool that'd list the most popular/forked/starred/etc C repos, and do a search for you in those should be a few minute task.