No, it's just as if you make Doom 3 mod. You can run the engine in debug mode, see what crashes it (for sure it needs player's model, player's def, player's material and skin, at least one weapon def and script) and add those missing things.
I don't provide any support, sorry. If I could figure out how to take original BFG and what basic set of assets should be present in order to load a basic map with player on it, anyone else can

As long as there is a desire to make it happen

The most difficult things like fonts, SWF menus, etc. are already there. So you don't really need to worry about all that.