@b10c runs a backup script for bitcoin-core and libsec256k1. These are pretty useful to understand the structure of the data and what to expect.
core developer Fabian also created a one way bridge to a self-hosted gitlab. 
se https://delvingbitcoin.org/t/gitlab-backups-for-bitcoin-core-repository/624
I remember finding a python tool that seemed to be quite active - can't seem to find that now. I'm not sure why Fabian didn't use it. Maybe it didn't meet his needs.