I rebuilt the starter today, the most difficult part was soldering the brushes onto the old wires since i never soldered before, but it worked quite well.

Now the starter spins a lot faster, and it is possible to start the engine on one battery

Next problem to fix: Huge cloud of black smoke out back.

It most likely is the diaphragm in the injection pump since i have the most common symptom of diaphragm failure, that is oil in the vacuum line towards the intake.

Not an easy job, but what choice do i have /: