i have not done much with this yet.. but i was able to create an empty tiledata.mul with this python script that will open in UO Fiddler and not throw an error.
the intent for this is to basically create the empty tiledata.mul and art.mul from scratch.. and then start loading totally unique art into them with UO Fiddler... then load that into CentrED# to create totally unique maps.. and then load those into Godot.
https://github.com/MichaelTen/TerrainLab/blob/main/blankTileReg.py