Model 50 electronic 6v ignition

This is a personal point of view, 6V electronic ignitions are susceptible to failure so I recommend 12V for the ignition and all of the bike/battery etc. Boyer Bransden do a 6V and I think Pazon do one, but Pazon is built in New Zealand so the cost??
The significant point you need to consider is how the pickup is fitted to the bike? Did the 56 Mod 50 have an 18D1 or another system with points when it came from the factory? You also need to understand the 6V battery charging sytem. If it is the original switched alternator system then this is inadequate for 6v electronic ignition. (which might have contributed to one you have failure).
12V is recommended.

... with Al. There's no good reason to stick with 6v especially as 12 volt systems are much better regulated without those awkward switch connections. My experience with the Boyer on the Electra shows that they hate low voltages.

...on the traditional 6V charge regulator system with the PRRS8 and switching of alternator output coils, the charge regulation is very poor such that you can get a momentary high 'battery/system' voltage which can kill the 6V electronic ignition. You have to remember that ALL electronics can die quicker than you can blink. 
The Electra is 12V to start with and the Boyer Brandesen ignition being upset about low voltage is purely a factor of BB not being built as good as it can be. Bear in mind we have had BB ignitions for many years, they are getting better and other manufacturers have made better electronic ignitions.

Electronics on vehicles with field excited alternators are designed to cope with transients of a couple of hundred Volts, which adds just pennies to the component bill. I doubt that the existing 6V classic bike ignitions have this feature and considering that most owners would go for a 12V conversion, market size probably doesn't warrant producing new designs. 
Another problem with 6V electronic ignitions is that the microcontrollers within them traditionally operate at 5V, so adding the dropout voltage of the voltage regulator in the unit, it doesn't take much of a supply voltage dip before it misbehaves. A design featuring a modern microcontroller which can operate at 3.3V with a low dropout regulator would provide more headroom against supply voltage dips, but I doubt that anyone will be in a hurry to produce one.

Why not use a 6v to 12v boost regulator, cheap as chips on amazon etc  Just to run the ignition!

Why own and love a 70Year old motorcycle and want to fit modern chines cheap parts parts? Might as well buy a Honda!

I didn't say you had to buy a cheap one, or Chinese!  You can make your own  It would just feed the ignition so no need to modify the bikes wiring at all
Regarding the pulsed load I agree  I found my Boyer igntion generated spikes on the 12v feed even with a good battery, cured with some capacitance across it