May I assume this is a Bren 805? I was not aware of factory California compliant Brens, but on an 805 I had, the stock folding mechanism was disabled with a woodruff key placed behind the release button. You have to remove the stock and drive the hinge pin out. When the stock is apart, the blocking woodruff key may just fall out, depending on how it was installed. Hopefully the release button spring was not removed, but if I recall, it is sold by CZ-USA.
As far as the magazine release is concerned, there were some aftermarket companies making California compliant parts such as Bullet Button or Raddlock, but my memory may be fuzzy. These are a pain to install and possibly impossible to remove as there was a pin you drive into the magazine release shaft and there is no way to drive it out from the other side. I suppose you can drill out the pin on a drill press or with a steady pair of hands.
Your best bet is to find an 805 owner who has swapped his AR-style magazine well to a European style "retro" magazine well with paddle release and ditch the Californicated magazine well. Or maybe you'd consider the European style mag well? But you will have to run HK36 style magazines.
You can also buy all the parts and put a new magazine well from the parts to be found here:
https://shop.cz-usa.com/SearchByKeyword?word=bren%20magazineBest of luck!
