This game was designed for 4:3 monitors, but it runs in 16:9 mode. Any way to set the correct aspect ratio?

Edit the toon.ini and under [scummvm] add the following line:

aspect_ratio=true

That should force the aspect ratio to 4:3 ;-)

He's telling you to change that line to Change Aspect_ratio=true to =false.
Or you could get some effect by overriding your display scaling in your GPU menu ie Nvidia control panel or radeon settings for AMD

Tried this out i have, No change for me.

just makes the game window small forcing me to maximize it.