Well backup your project first before trying any of this.
If you open the c3p file with a zip program and open the .proj file has the version that you can change to make it so you can possibly open it up in an older version of c3. If you didn’t change your project since then you can see what version you used then. Chances are high it won’t work though if your project uses any new features.
The second part is opening up an older version of c3. You can specify the version in the url. You’ll need to try out the various versions to see when the c2runtime option was removed or search the release notes.