I would suggest adding a relay and cleaning and keeping your switch. The relay takes much less current to activate yet will supply full current to the ignition system.
If you add another switch, it will be in addition to your existing handlebar switch and not an easy reach from the handlebar grip.
If you are doing away with the entire switch and changing the master cylinder, then you can use most any Japanese or Italian switch gear that uses a normally closed kill switch.(I like using the late model Ducati stuff. There are usually a number of them on e-Bay)
Don't add a typical British magneto kill button as these are normally open and push to ground. This will cause a direct short in a battery and coil system.
I have worked on this project for a long time getting the fitments to apply to Norton, BSA and Triumph maybe others. It is a three relay kit with the required wiring to put it on your bike. The relays are Motorcycle duty not car relays that I started with. Right away in the development the problem of models with small head lights came up so an under the tank system was chosen. The use is to your digression but started as high beam low beam and horn. On older bikes 67 and back with grounding switch horns I have used a separate relay wired it backwards in the headlight just for the horn and then used the third relay for the ignition switch. The kit includes a 90 degree H-4 headlight plug, a fused connector for the battery end, the relays are pre-wired. All the wiring, shrouding, and connectors are of the highest quality. You just wire them on to your handle bar pod switches in the headlight shell than hook up to your Battery. Nothing shows but now the light and the horn really work well because the voltage does not have to travel up into the pod switches and than back to the device powered. Only enough power to trip the relay via the switch is needed after fitting the kit. PM me if you have interest in the kit.
norbsa 1960 TR6 1963 Super Rocket 1965 650 Star 1966 441 1968 Thunderbolt 1969 Twinkle 250 1972 Fastback 1974 Roadster 1970 S.S Way too many BSA's not named http://decentcycles.com
With all due respect to the two posters above, from experience, I wouldn't use a relay switched by the kill switch to supply the ignition.
Bike makers have used relays on bikes for at least 40 years - switched by the handlebar horn button to supply twin horns, switched by a handlebar lever to supply flashers, switched by a handlebar button to switch an electric start, etc., etc. In addition, I've used relays on bikes for over 25 years - switched by the dipswitch to supply a high-wattage headlight bulb. The one thing all those have in common is, if the relay fails, it isn't a disaster.
Otoh, if either the kill switch or its relay fails - say, in the middle of an overtake with a truck heading towards you (this is known as Murphy's or Sod's Law ) - it is a disaster. Ignitions draw a relatively low current - points or first-generation electronic (e.g. Rita or Boyer-Bransden) about 3A, modern e.i. a little less - so adding a relay takes little load off the switch, but adding a relay adds a potential failure point.
Fwiw, I've run Brit. bikes with kill switches that have a distinct 'on' and 'off', Brit. bikes with kill switches that have to be pressed for off and Jap bikes that actually earth the ignition power through the kill switch ... and, ime, neither one type is any more, or less, reliable than any other. Just buy a new switch, maybe open it up and check that it doesn't have a manufacturing fault that would cause only a tiny contact area that will burn through, fit it, maybe check that there isn't an unexpectedly-high current draw across it, then forget it except once a year or so when you give it a little tlc.
I would use a single pole double throw(five pole) relay, with the ignition wires run through the pair of normally closed contacts(30 and 87a).You kill the ignition by energising the relay(contacts 85 and 86) with 12 volts from the "extra" button on the other side of the switch gear. Wired this way, the relay contacts seldom move and theoretically should last a very long time. You could even tee the unused terminal(87)into the brake light circuit to warn following traffic that you will be slowing down.
Thank you all for the advice. I'm the type that likes to keep things as simple as possible so I'm leaning towards using the type of kill switch that my old Rocket 3 had mounted on the handlebars, the push-to-open one with rubber boot cover like the rear brake light switch. For now I've just bypassed the stock switch by removing the two wires from the circuit.