Squareback vs round back alternator?

Thank you all for your feedback and input, now we have something to digest and test.
I have found a transistorized regulator (like the one @Mattax showed) and to my surprise someone in Berlin sold it via ebay, it'll arrive very soon. But before we install it we need to draw a clean wiring diagram and eventually modify the wiring because it's all messed up. The original wiring is very simple but the past modifications (e.g. additional fuses, relais etc.) makes it more difficult to determin the fault(s). I'm gonna keep you updated and sure we have more questions to post soon.

Thanks again!
Christian