The voltage regulator can be swapped without replacing the alternator. For a Valeo alternator (I'm pretty sure the alternator on Z4M's are all from Valeo) you can replace with Valeo part number 593873. That is what I used when I changed mine. I did it proactively before troubles started to appear.
https://www.ebay.com/itm/22237535128...0AAOSwnHZYdwNy