These are becoming a bit hard to get hold of, in working order. A problem, if originality is important. Otherwise you can use the Nosso regulator available new. It is a version of the Bosch regulator found on Goggo, Isetta and such. However it would work for you. It is not going to go straight into a Black Box however, as it is a wider design, as was the bigger Bosch item it copies.
There are several people now offering a solid state version of the regulator, placed inside an old regulator unit so it looks original. I have never used one and would make no recommendation of the units. I prefer, and have original stuff still. They are more expensive that the Nosso, in my experience. Though there are folk selling Nosso at silly prices. Trust this helps.