The magneto alone is not much of a problem, usually If the coils and condenser are NEW. Old ones will let you down. They don't like being hot is the first sign of impending failure. The "automatic" advance and retard is something I don't like. An expensive item that MAY not work as it should when it is aged.. I would prefer a manual operation for simplicity.. New ones are being made however so..
IF it's a magdyno it's a big item for attention if you consider the whole item, as a unit. The generators only ran about 8,000 miles between thorough checks and the slipping clutch is another item requiring attention or it will fail. Having said that, ALL my ARIELS are still running the standard set-up down to the original voltage/current regulators or 3 brush as original.. Apart from having the coils rewound I do al the work myself as there are a few "repairers" that should be doing anything but motorcycle electrics. IF your electrics are still unmolested and you know a reliable repairer stay with the original unless you run at night, where you might need better lights for safety.. IF you don't have any rebuildable originals It's possible that the alternative offerings will do the job, and do it well, but you would need to do your own research. If the article is not supported later you actually are in no man's land. Nev