Boot both machines via usb†, start ssh server on one

Depending on where the ssh server is (source or target):

    dd if=/dev/sda | ssh -C 1.2.3.4 dd of=/dev/sda
    ssh -C 1.2.3.4 dd if=/dev/sda | dd of=/dev/sda

† or mount the source / as ro, or systemd-switchroot to a ramdisk. Just make sure that source / is not getting writes. Or not, if you don't care.

I presume, something like...

    ssh old_mac "dd if=/dev/sda" | dd of=/dev/sda