There's an outfit right here in my backyard called Portland Engine Rebuilders that offers an upgraded setup that replaces the factory "upholstery" buttons.
They sometimes have sets made up, but I just called and they do not have any sittin' around.
The price I got was $140 for a complete set of four rails (one engine's worth). You send yours in, and they install steel rocker guide buttons that are riveted in place. Turnaround time is 2 days.
Their phone number is 503-230-1276.
I personally didn't really see a need when I screwed my engine together. I mean, there are thousands of these engines running around with the nylon guide buttons. I think that if they are replaced before they get brittle, that would greatly decrease the chance of having a failure.
I've seen P.E.R.'s setup, -and it looks nice. I'd definitely put my money on a rivet over a small piece of barbed nylon, but on the other hand, -it IS hard to argue with so many stock setups running around with no issues.