The SWH FAQ[1] seems to suggest that you can download a bare git clone with the "revision" download option, but I can't seem to find that option on the portal64 repo.
[1]: https://www.softwareheritage.org/faq/#42_Can_I_clone_a_repos...
[1]: https://www.softwareheritage.org/faq/#42_Can_I_clone_a_repos...
https://archive.softwareheritage.org/api/1/vault/git-bare/sw...
Once it's finished, it should be available at:
https://archive.softwareheritage.org/api/1/vault/git-bare/sw...