Hellhole Island (original) source code for version 1.6.0.3. Decompiled using dnSpy. Only includes script files.
This repository therefore CANNOT be used to recompile your own version of the original Hellhole Island, but it MAY be used to create a modified version by replacing the game's Assembly-CSharp.dll with your own version. This has not been tested, so you might need to open the original file with dnSpy and make changes in it.
As for the question of publishing the entire project, that is not possible (as far as I know) as I got rid of the original source a long time ago, and that happened long before I started using VCS (GitHub) for my projects. However, in the future I may publish a separate repository with asset files extracted from the compiled files.