2 replies in one:
Back then in the early 80's, as far as I know, the only generally available after-market electronic ignitions were Lucas Rita and Boyer.
Lucas Rita had been fitted as standard to the T140 since about 1979, I think.
Both these systems were intended for replacing points in timing cover (ie unit construction) bikes.
As mine is a pre-unit T120 with magneto, I couldn't use them in that way.
Fortunately, an engineering firm called Mistral Engineering started making a housing which fitted in place of the magneto (holding the magnetic pickup), supplied as a complete kit with amplifier box, coils etc, for much less than the cost of having my duff magneto reconned.
It was a great setup, giving great service for nearly 30 years, with only the coils needing replacement once, and only met it's maker due to overvoltage when my zeners went open circuit, toasting the amplifier box.
Lucas stopped selling the amp ~10 years ago, they can be reconned (at a price) or bought used, but I didn't want to be reliant on these solutions.
So I bought a magneto replacement from Norvil (also available from Goff, Pazon etc) which is able to take to take any of the side points replacement electronic ignitions, and stuck a Boyer MK4 in it. It has worked fine for a year or so, though the mag replacement body is not as well-engineered as the Mistral/Rita unit was. I am hanging on to that old unit, as eventually Mistral will be supplying their high quality Moira amplifier, which will run off the original Rita pickup/mag body.
To loxx101's query, the 3-phase Boyer powerbox doesn't "supply" as such, it just regulates the alternator output. It is rated at 280W, which gives a generous safety margin for the Lucas higher 180W max output alternator (there is a lower ~135W Lucas 3-phase alternator also available, confusingly often listed with the same P/N).
My experience with the higher power alternator was that it produced enough power at low revs to enable a fair tickover, with no power being drawn (lights etc) other than for the electronic ignition (somewhere in the 35W ballpark), but you would need a capacitor in place of the battery to smooth the charging pulses. I believe the Lucas 2MC is the job, and is listed by Norvil.
While this got me by for some years (it's a T120 in a triton, and I didn't have anywhere to put a battery at the time) I did find night riding tedious, either having to turn the lights off at junctions, or hold the throttle at a fast idle to keep lights on.
Ultimately I glassed a battery pocket onto the front of the oiltank, and run a battery ever since.