Where is the source code for Ethereum itself, not the Go or Js implementation but the Ethereum itself, including EVMs, for a complete copy fork of Ethereum. Bitcoin can be completely forked, but Ethereum cannot be found, so I don't think this is open source.
I don't think there is s a single repo, as we all know there are a lot of implementations of Ethereum since the beginning. As you have said, it has a Go version or Rust.
For a complete implementation you may want to visit this site, https://ethereum.org/en/developers/docs/nodes-and-clients/#clients. And hopefully you can find what you are looking for.