The Bantam had its origins in an earlier German design, the DKW RT 125, I don't know if these bikes had an alternator or not, but maybe worth researching this bike and also other pre-war designs.
There was also apparently a Watsonian-JAP designed around 1948, it used an all-light-alloy JAP 50-degree side-valve twin of 998cc, producing around 35 bhp. and was possibly the first motorcycle engine to dispense with a Magdyno and, instead, incorporate an alternator unit. The alternator was housed in an extension to the timing chest, and from the rear of the chest there projected a car-type distributor. There was a rectifier and battery
and, coil ignition. See This Link
for details, the bike is apparently housed at the National Motorcycle Museum.
Lord knows who made the alternator & distributor, though Lucas
/WIPAC etc are likely suspects.