What Gates Carbon Drive parts should I upgrade? A friend gave me his old Trek District 1 bike, which has a Gates Carbon belt drivetrain. The bike's about ten years old.

The equipment


an old-style (pre-centre-line) CDC drivechain
a splined (single-speed) free-hub


The problem

The drivetrain is extremely unreliable and annoying. 

However much it's tightened, the rear sprocket (CDC 22T) eventually unwinds the ring that tightens it on the splined free-hub. The nubs on the sprocket that engage with the splines are worn, but whether this is the ultimate cause or effect of the loosening I can't say (the sprocket definitely needs to be replaced, anyway).

Below you can see the step-shape that all the wide nubs on the sprocket have been reduced to.



No matter how carefully aligned, the belt squeaks and rubs on something or the other (this is why my friend lost patience with it, and I've spent a lot of time messing about with it too).

What parts should I replace to try to rescue this bike? Do I need to replace belt, sprocket and chainwheel, or can I start with just the first two?

And, which of the new versions should I choose?

The replacement parts are pretty expensive, so I'd like to avoid wasting a lot of money on a futile repair attempt.
Hard to give a good definitive answer to this. It's impossible to predict of replacing one or both sprockets and the belt will solve the rubbling and squeaking problem, although it's possible that upgrading to the newer type Gates belt would help (presumably they have changed the design because it was better).

If the rear sprocket retaining ring comes loose, something is probably worn badly, probably by riding when the ring came loose the first time.

The retaining ring really needs to be tightened to the proper torque specification with a torque wrench, which is may be way more than you are estimating.

If the sprocket spines are damaged the hub splines may be damaged too. I'd evaluate the hub before deciding how to proceed.