Not wanting to risk massive engine failure, but not wanting to replace more than necessary, I am getting down to the items that I feel need replacement. I am now leaning to replacing the exhaust valves and valve springs, along with the rocker arms. Also considering replacing the timing chain. These appear to me to be the weak link.
I am quite certain that the engines all run lean. This increases the temperature of the exhaust gasses. The extra heat during long, fast rides weakens the exhaust valves, causing them to break at the point where the stem joins the head. Fresh new exhaust valves should last another 50,000 miles.