Thanks everyone, this game deserves all the love it can get. I should have done this years ago, but never looked closely enough into it.
The NES version featured here does have one of the expansions included, Return of the Witch Lord. Looking at the source, the level layouts are pretty straightforward. It doesn't look too far fetched to modify the levels into other expansions. I've never completed all of the levels, so I haven't needed additional content. Hopefully other fans will take a look at it, too.