Meynaf has finished 3 ST to Amiga conversions, so I'd say there's nobody with more knowledge when it comes down to it. If the problem with it really is only a control problem, then Meynaf could probably help with it.
The only things Galahad has mentioned so far is a slow screen update and a control problem. So is it 90% complete? Or is it actually more like 10% complete and there's a flood of bugs and unpatched Atari ST code still to adapt? Galahad could clear this up by posting the demo or say what percentage the game is at.