Hi Paul, please learn to walk before you try running. Machine Code is awesome, but I suggest you try to understand the hardware a bit more first. This game is a good starting point as it uses SCREEN 1. In this screen mode, you can define a custom character set, and that's what the game does. Maybe you can experiment a bit with that.
Go to SCREEN 1 and use WIDTH 32. Type the FOR NEXT loop you see on the picture and press enter. Now you can VPOKE characters and colors. Give it a try and see what happens.
VPOKE between address 256 and 2047 to modify characters
VPOKE between address 8196 and 8255 to modify colors (8 at a time)
Technically the start addresses are 0 and 8192, but I skipped the first 32 characters of the character set, because you can only print them indirectly. And you don't need them for this experiment.
LOL Maybe i'm just dense 
For me your English is good but back to front in some cases. For instance you wrote -
You advance automatically once you picked up all chicks and picked up the minimum number required. Iirc this number is in the level data. I think more for each level.
This is factually correct but its better written as -
Once you have picked up the minimum number of chicks for that level and then, having cleared whatever chicks remain, you then advance automatically to the next level. You will only advance once there are no chicks remaining to be rescued on screen so you will have to be fast before more are dropped !
The first sentence is enough but the second sentence is there to emphasis the point and to get the need to clear the screen before progress can be made across to your British neighbor !
However I can barely string two words of Dutch together so you have done very well in comparison to me. I think there's a little lost in translation between the differences in how we think.
Ive tested the game and can in fact advance once ive cleared the screen successfully. I was looking for a bug where there was none !
Thanks Manuel.