A local Retro-Modern bike shop in West Seattle.    RideYourBike.com    

Twist Shifter (Gripshift) Stem Mount

We can make this for you for $40 if you cannot DIY.

Used or community bike shops are perfect places for finding the brake lever mount.
Chicago Schwinn bikes like the Varsity used 7/8" (22.2mm) handlebars. Usually the brake levers are a dime a dozen!