The aluminum idler gear was an aftermarket item that was sold from the late fifties until early 80s. Apparently quite a few were installed at rebuild time. As you say this was a problematic part. Even if they did not fully strip, they would shed metal into the bottom end and cause serious problems. My OZ Vincent had one fitted when I got it, I tossed it immediately.
The original Vincents as delivered were a very good machine, unfortunately some of the aftermarket parts used along the way have caused problems.
I have the original series of MPH (the monthly VOC rag published 1949- now) in which Tony Rose did a 100,000 mile road test of a Black Shadow with Steib sidecar and reported on all maintenance required. He used the bike for daily year round commuting as well as holiday travel over a four year period. They pulled the heads for decoking at 50,0000 miles, other than that it was all just oil changes, new tires, chains and sprockets and so on.
This was quite an accomplishment for a motorcycle in the early fifties. I dont think too many other machines would have held together so well, especially when heavily laden like this one routinely was.
With a few modern items, these bikes are great for long distance touring today. Three of my friends just got back from a 3950 mile trip from Vancouver to Colorado, sidetrips and back. One bike had a minor problem with a new style alternator that had been fitted, that was the only problem in 12,000 miles of total travel. We do these mileages all the time on them, trailering the bikes is really frowned upon. And the guys I ride with really roll, they do not baby these bikes.
Glen