Why keep `main` in C? That's where you should start the "incremental" rewrite. Have the main in Nim instead and call the C code.
But it also depends on the project. Many projects esp if written in C are only big because they are written in C. The C code is an endless repetition of malloc wrappers and manual memory management and manual error propagation, it doesn't do much. You then don't have to "incrementally" rewrite it, just recreate your project in Nim with native Nim libraries.
