Reporting back - 4 weeks have passed and I've learned a lot. Since I'm a perfectionist at heart, I didnt want to buy a replacement motor box as the gears would be either broken already or break some time in the future for sure. So I decided to machine replacement gears from brass
...oh boy, what a huge project just to fix an air recirculation flap
I had never made any gears before so this took a lot of figuring out. My dad is a precision mechanic and has a very well equipped workshop incl. lathe and milling machine at home. I didnt go for perfect
gears with involute teeth, as I didnt want to buy the special cutters needed (I was surprised when I asked my dad and he told me he does not
). So I simply used small saw blades on the mill (0.6 and 0.8 mm), cutting the gaps between the teeth with 3 cuts each, giving a simplified but sufficient tooth shape. I lathed down the original plastic gears, removing the broken teeth and making room to mount my replacements.
So today I've put everything together, closed the motor box and mounted it back to the fan unit. And the flap works like a charm - up, down, fresh air, recirculated air, perfect!! In the end I decided not to exchange the resistor as mentioned in my last post. After seeing and hearing the flap in action it was pretty clear the motor power is already close to the minimum required.
So there you go, another possible way to solve this issue. Now I probably own the most durable air recirculation flap motor box on the planet that will outlive the car by decades
If anyone is interested in this quality fix, let me know. I could machine more parts with relatively little effort since I got everything figured out now. Work takes around 6 hours for everything though since its not CNC.