changing your clutch to a Triumph 4 spring
an electronic regulator in place of the buzz box.
two way damping rods in the forks
I don't have X-ring chains on my Gold Stars so I don't know if they fit (they're wider than a standard chain). If a #530 X-ring chain does fit the only advantage of the narrower #520 would be the lower weight of the chain itself plus the narrower sprockets. It is several pounds of mostly unsprung weight so there is a performance advantage to this modification but it comes at the cost of a fair amount of effort required to achieve it.
I had no choice but to replace the clapped-out shocks on my Catalina and went with Hagons with shrouds because I wanted the bike to look original. The Hagons came with progressively-wound springs so they should feel better than standard springs but, having said that, I have no complaints about the old Girlings on my BB for the riding I do that doesn't push the limits of performance.
If you read this thread
you'll see that there are plenty of places where an old 6-spring clutch would have suffered wear and abuse that no doubt contributed significantly to the bad feeling some people have toward it, but the 6-spring clutches in both my BB and Catalina, rebuilt as described in that thread
, work great. The clutch levers have the same pull as on modern bikes, they don’t slip under power, and they free up completely allowing neutral to be selected when stopped. Since that's all I can imagine needing from a clutch I don't know in what way a Triumph 4-spring would be "better."
I enjoy electronics so I like the challenge of making the old electromagnetic regulators work properly. That said, when I was wiring my BB (which has a magdyno), and before I decided to go with a total loss NiMH battery
pack with LEDs, I bought a Podtronics
regulator for it.
As for the forks, my recent 1200-mile trip was the kind of riding I like to do and nowhere on that ride did I ever feel the need for two-way damping. However, this certainly depends on how a bike will be used. I have the dimensions of the rods and damper assembly so easily could make a set myself. Although I've postponed rebuilding my Catalina's forks for now, and at this point don't plan on modifying them, I'll revisit that decision when it comes time to work on the forks.
I would add brakes to your list since this "performance modification" will be responsible for keeping you alive. Most of the braking power is from the front so you should consider having modern shoes for at least that brake, arced to the drum, at the top of your list. I've had Vintage Brake do the brakes on at least four of my bikes, most recently the Catalina and Ariel. Although not cheap, an Eddie Dow TLS brake is period-correct and worth considering for your list.