Check the tension of the alternator belt.
Check the contacts ”generatorkol in swedish”. They are in the rear part of the generator. You have to disasemble the generator see them.
Old electrical system tends to fail because of oxidized wires and contacts causing voltage drop. Test with new cables.
You can do an easy check on the regulator just by removing the triple kontakt on the regulator and connect a wire between D+ and DF on the triple contact. Now you have maximum output from the generator. Max rev is 2000. With a correct working regulator you should have about 14 volt. Without the regulator the voltage should be higher.