Cranks last a long time, 30,000k's is the "service interval", plenty go past that without trouble and the age of the middle seal (it's replacable on the MC21 and 28 but not on the '16 or 18 ) seems to give more problems than the bearings themselves. Run it on good oil and be nice to it, don't for example coast downhill with the throttle closed. Dip the clutch a blip the throttle a bit, that's the only time it gets any oil, the rest of the time it's spinning dry.

Barrels don't wear they just get damaged, so in theory they just about last for ever, a decent 2nd hand original plating Honda barrel would be my choice over a fresh replated job every time, if it costs £100 on ebay and comes with a piston, rings and a little end all the better.

As for rings, well, for most of us it's every time it seizes or drops a horse or two on a dyno day, except for Steve who does his every other Monday.
But IF you manage to run it just as a road bike and leave it in completely standard form the service interval is 10,000k's, I believe Leigh's '28 roadbike is way way way past that as Leigh like most people is reluctant to rip down an engine that's running well.

But (in my opinion) it's also very important who does the work, I'd advise do it yourself, yes you'll make mistakes, yes it will cost you money (but not as much as a mechanic will charge) but it's all a learning process and in a couple of years time you won't even bat an eyelid when the engine needs to come out.
But if you choose to let someone else do it make sure they know these bikes inside out, too many tales of wrong crank bearings, bit's left in crank cases, wrong heads on barrels etc. are on this site (and we're the people who take an interest in our bikes) after all if someones going to learn how to rebuild an NSR with your NSR, it may as well be you.
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