Just for the hell of it I grabbed the Atari ST version and took a look at the game file (75k of mostly code which is rather big for a game).
Now I'm no expert but I reckon it would be quicker to completely rewrite the game from scratch on the Amiga rather than trying to understand the code enough to be able to replace all the graphics and sound routines.
Galahad: If you have also pulled apart the main exe I would be interested to see if you came to the same conclusion. From my 5 minute analysis it appears that the game has not been written with any thought of conversion to future platforms in mind. But I would love to be proven wrong on this!