One easy way to tell is to make a new game with your version of Ultima III. If your party starts with 100 HP and 100 Food then it's the original version and won't work with this scenario properly. If you start with 150 HP and 150 Food then it's the newer version and this scenario will work fine with it.
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Yeah absolutely. I play Ultima III in DosBox, most people do. The GOG version of the game runs DosBox.
All you want to do is make a backup of the files in the Ultima III directory, then copy my files into that directory overwriting the existing .ULT maps and EXODUS.BIN file.
When you start the game you'll still have the original title and main menu, but once you've made a party and entered into Sosaria you'll see that things are very different.