I finally figured out a simple solution for Ratty's brake mechanism. I decided I didn't want a foot operated rear brake, as I wanted some flexibility in where I mounted the footpegs. And since I don't plan on running a front brake, I decided to make the normal front brake lever operate the rear brake.
I had several sets of spare cable, some intended for brakes, some intended for clutches. I think I ended up using a spare clutch cable off a CB160. I had a piece of aluminum casting in the junk drawer that looked just right for locating the end adjuster piece on the outer cable sheath.
Cable attaches to the Trail 55 rear brake arm:
Small bracket made from spare aluminum casting found in junk drawer:
I also got the air cleaner/horn mounted on the carb:
I decided to keep the main body of the horn in raw fiberglass, and give the outer plate the rust treatment. Small wire mesh keeps out the bugs: