A new ignition switch is about 16 bucks plus shipping from denniskirk.com. Part # 20-211.
A local auto store would probably have them, too. Basically the same switches John Deere or Cub Cadet garden tractors use....almost a "universal" key switch.
You can use a 2 position toggle switch, with 2 spade terminals and have RUN or OFF....but you would never have a brake light.
On a 2 spade toggle switch, hook the gray magneto wire that goes to terminal M on your current keyswitch to the top terminal, and the black ground wire (G terminal on your current keyswitch) to the bottom terminal.
Toggle switch up to run, switch down for off.
You could splice the 2 wires together that go to the "L" terminal on your current key switch and regain the function of your brake light....one of the wires should be a red/yellow wire and the other a yellow/red wire. Surprisingly, splicing those wires together that go to the brake lever switch also lights up the Tachometer light (all the time)....but that is the only light that would be on.
To have the ability to switch from "off" to "run" (which includes the brake light always ready to function) to "run with all lights" would require a 3 position toggle switch with 3 spade terminals....plus you would have to splice together several wires to make it function properly.
I could describe what that would require if you wish.....but a $15 new switch sounds better and better.