Well, I've been playing with it some more. It is true that at 320x240 it is almost unplayable. Almost all of the windows take up the entire screen and some are even larger (Main menu, Financial report, Settings, etc.). So, I've set the resolution to 640x480 and then resampled the output to 320x240 using simple averaging of the four pixels into one. This makes all texts unreadable, but that can be circumvented by using a larger font (this also enlarges the windows).
It works quite fast and with a few optimizations, it will be even better. Now I am trying some fonts to find the best one. Darker text colors also look better, so maybe changing the white/yellow text into blue/black will do. Also I'm working on the controls, but soon it will be playable.
Some screenshots of the game, but it looks much better on a Dingoo display:
Nevermind, I forgot to turn on antialiasing of the text. It looks MUCH better now.