Hi, apologies for the unclear licence. I find CC licenses a bit clunky, so will clarify below.
The engine is under a BY-NC-ND licence. It is free to use, but only noncommercially, with attribution, and no derivatives may be reshared; if you make modifications to the engine for your own private purposes, that is fine. This basically translates to "no publicly released forks".
For games made with the engine, you may use it for free, noncommercially, and you may modify the engine as you see fit, just leave attribution.
If you wish to use the engine commercially, please message me via the contact form on my website to discuss. I do not consider itchio's default free-with-optional-donations to be "commercial" in this case, that's fine.