If you wanted to go really retro and make the music MIDI-based, you could theoretically reuse some of the music from MI2 seeing as that's where a few of the tracks in the game are from.
And you'll probably get a C&D anyway even if you don't charge for it. LucasFan's Maniac Mansion Deluxe was shut down I believe. I can't remember if his unofficial sequel The New Adventures of Zak McKracken was as well or not.
"Booyah! Look out, LeChuck! Here comes Guybrush Threepwood's glowing sword of hot monkey vengeance!"
-Guybrush Threepwood, Tales of Monkey Island