To not be picky, this is the moment when you should come into the project. This makes it possible to you to have a voice to fix the parts are not working nice for you. Also is your chance to learn C++, which is not such of a bad deal. C++ is not over-engineered if is written "correctly" as C is not either. Bad C coding practices exist, and bad C++ ones also.
Originally Posted by IanS
At the end, you can contribute just a small part in C, and people will rewrite it in C++, so still is no issue for anyone (if you don't want to touch C++ at all).