You are a lucky guy!!!
I can´t really answer on your question, but I make you hope: When my Farfisa organ once stopped to work, there was nowhere around repair service available. And although nobody has had electric circuit plans of that organ, a local organ dealer with his professional electronic engineer in the service department could fix it: There are so much electric pieces inside, and so few ICs, that you do not so much dependent on original spare parts, but only on finding a professional electronic engieer with a proper multimeter and a soldering rod. So, check some local organ dealers´s service depatments and ask them for their quotation: I am pretty sure that the service engineer will love to have the honor to fix a B3!
learning Piano on my Roland HP-505
before playing Drums in adults bluesband on handpicked set; before crashing E-Guitar in kids garage band; raised on home entertainment Organ and Keyboard models Eminent Solina P240, Farfisa Maharani 259R, Technics KN800, and on Mouth Organ, Recorder and Accordion