Suddenly the needle of the original Jaeger "benzina" gauge got stuck. It was not an electrical problem as I though on a first glance but the potentiometer strip inside the in-tank sender had reached the end of it's life. As a first aid I repaired the broken wire with a constantan resistor wire (see next photo).
Later I dismantled an old fuel level sender and made a repaired new one out of the two old parts. The potentiometer housing can be opened easily by bending three sheet metal clamps.
Now everything works fine again. It would be wishful to have some more damping at the needle. To achieve this the friction between the potentiometer pick-up axle and the sheet metal plate (electric ground) can be increased by fitting a small shim. The eight screws on the flange are M8 x 12.