Or, if you like keeping things looking original, but want better performance than the old selenium diode rectifiers, go to
http://www.olypen.com/retro/ and get the replacement that is virtually identical to the original. (part #31700-259-030) It uses modern diodes, so it's not subject to the failures typical of the original after 40 years, plus it hooks up identically so there's nothing to figure out. It's 40 bucks for your bike, but the best choice in my book. I just recently put one in my CB77 after a sudden mystifying electrical plague that turned out to be the old one suddenly giving up the ghost. Fine one week, sputtering, choking and fouling the plugs the next! Now I have no problems what so ever.