If I remember right from my Bantam racing days, someone did a 5 or 6 speed Bantam with 2 gearboxes coupled together but it never really went all that well. Someone in the late 60's also did a rotary valve Bantam. Pity the factory never took any notice. Tony Jones finished 8th at just over 83mph in the 1974 125 TT race on a 3 speed Bantam, made to the Bantam racing clubs rules, against much faster opposition with twice as many gears. Then, of course, there was Mick Scutt and Dick Hunter who also rode Bantams in the TT very successfully as did lots of other riders.