I bought an electronic regulator from Zofia’s Oldtimerelektronik shop on ebay.
This regulator works for the Bella. Here’s the installation manual:
You can mount in on the original Bella regulator plate (please note the photo is from e-bay, mounted on a Lambretta plate), but I would suggest you order m4 x 40 plastic screws and nuts to go with it. You can use them to fix the regulator to the plate without causing a short. The regulator comes on a plate with three holes that line up exactly with the Bella’s two bottom feeds and the hole in the middle of the plate.