Thank you very much, Zac, Alexis, Warren and Tim !
In the meantime, I have painted more small parts and prepared them for installation in the cockpit. And then I tried out bracing the fuselage frame with the elastic thread included in the kit from Modelkasten. The kit also includes very delicate turnbuckles, but I made my own from stretched cotton buds and painted them white. In the original, the turnbuckles were probably wrapped in fabric.

Now the compass, control column and seat with seat belts, painted with oil paints, are installed. Lead wire was added to the control column for the machine gun trigger and auxiliary throttle cables.
For the control cables, I used 0.1 mm rigging cord, which is used by ship model builders.

Next, the remaining tensioning of the fuselage frame must be done, and then the whole thing will be installed in the fuselage.
Cheers
Joerg