The problem with ScummVM, I think, is that it's meant for modern systems. It's probably too big to compile for or run on a Turbografx. Porting it would mean more than just using the existing source code--you'd have to rewrite big parts of it and only use the bits and pieces of it that were meant for the simpler games. Of course it depends on what you mean by a port. I doubt that Mysterious Song uses any significant amount of code from the original Mysterious Song, rather than being rewritten from scratch.
Also, any game I'd want to see a port of is probably one I've played to death, and therefore wouldn't care about a port of, unless it's something like ScummVM that runs many different games. Maybe you could try porting Frotz? (Which is an interpreter for text adventure games such as Zork). It's been ported to the GBA so the system requirements probably aren't too bad (though you'd have to think about how to support a keyboard).
Maybe you can try porting the Palm version of Jumpman Zero (
http://www.jumpmanzero.com/) which does have source available, supposedly, although it's not in an obvious spot on the website.
