The C64 cassette version actually uses completely different code to the disk release, as that code could not be compacted enough to function from cassette. The underlying code comes from Terrormolinos under license from Melbourne House.
Like the questprobe game "Spider-man" this game has the same issue with running under windows.
The game can be hacked to run under windows by using a hex editor and changing these bytes in the file FANTFOUR.EXE
Offset b2ae: change 3F to 35
Offset b2af: change 3F to 33
Offset b2bc: change 3F to 50
See the trivia under spider-man for the explanation of these changes.