After a good few evenings work reverse-engineering things and applying fixes, Sympl now builds and can be installed, however, it's still very much in an alpha state, so it shouldn't be used in a production environment!
If you are feeling adventurous, then you can install it on a clean (disposable) Debian Stretch Virtual Machine, by running the below as root:
[instructions removed as they are no longer needed]
To make it very clear, do not install this on a server you want to keep, and ensure you have full backups before you do!
Next step: Automated testing!