Hello!
I feel that the main problem on the way of changing the game sources might be that the game uses a custom binary module which at this stage of development is built in the Godot engine, see /godot/modules/ignition folder in the repository.
If you'd like to use a stable version of the game for playing around with, please use the HEAD from the master branch. But in this case you might need to build the Godot-3.5.2 with the custom module included which is in the https://github.com/z80/ignition/tree/master/godot folder.
If you are looking into a development snapshot, please consider dev branch. In this case it is Godot-4.1.2 in https://github.com/z80/ignition/tree/dev/godot folder.
For the latter one there are Windows prebuilt binaries available in https://github.com/z80/ignition/releases.
Please let me know if you have any issues.