If done right I think that rear disk would be a nice upgrade for the ol' CR
I hate the rear brake on my 84 not because it lacks in performance but because the aluminum arm is a rock magnet and gets bent frequently.
With machine work fork upgrades are unlimited. A popular swap is the 87 CR cartridge forks. They swap over easily and have a reputation for being one of the best forks ever made. I have them on my 84 500 and couldn't be happier.