Regarding a tutorial, I think the best solution is to create a new HTML manual which explains the original game as well as introducing new OXC features. X-COM is an old and complex game that wasn't made with a tutorial in mind (even XCOM2012 had trouble doing that) and, therefore, I believe there's no point in attempting to shoehorn one in. After all, X-COM remained popular all these years without a tutorial, so do why would we need one now? Let's just provide a better and more readable manual for newbies, translate it, and be done with it.