So it's not the game's fault per se, or anything in its settings that would affect. Only thing is that it could mention in its documentation which resolutions it requires.

You'll have to tune the Monitors preferences in MorphOS to get the mode working and hope that it's not your monitor (hardware) that fails to scale up these smaller resolutions properly.

The game uses the "8Bit 640 x 480" mode, so select it from the Mode List (or create a new one if it doesn't exist) and start editing it. In the Edit Mode window there should be several pre-defined 640x480 modes to be selected (60, 72, 75 Hz or such), select one and click Test, repeat with another mode if the display isn't good. Hopefully one of the modes would work, and then you could save it and the game should also work. If no pre-defined mode works, a desperate step is try to tune values manually... maybe checking the monitor specs would also enlighten something...