I have played about 2 times on an organ yet, but I prefer piano.
The Organ Prelude from Monkey Island 1 sounds pretty good on an organ and
the MI2 Cemetery and Swamp music as well.
I'm glad to see people visiting the GF site where our sheetmusic is shared to try them out, too.
You're right, a few songs (MI1: Fettucini Brothers, Intro; MI2: Captain Dread, Stan, Swamp, Woodtick) aren't on the site yet because GF is on holidays.
You can find the remaining sheets on the site when he's back (end of this week), I hope you can wait that long