These engine are capable of going round the clock twice - bottom end. When talking to Motorworks some years ago, they stated that they would never bother to strip a bottom end because of this - it was just not worth the bother or the expense - there are always plenty of serviceable bottems ends available. Generally they are rock solid.
Nickasyl Barrels - life of the engine. Pistons should not need changing - but rings might benefit a change half way through engine life.
Oil Pump - life of the engine.
Connecting rods and bearing - Life of the engine
Crankshaft main bearings - Life of the engine
Flywheel seal - do it when you change the clutch - you might get 100,000 miles out of a clutch - by that time the seal is probably weeping.
Gearbox bearings - gearbox is the weak area. But it all depends on what sort of abuse its had over its lifetime. Some gearboxes go for life of engine, but many do not. Depends on water ingress and insensitivity of the rider. And luck. Should see out 100,000 miles though - enless you like wheelies.
The thing to look at is the top end, as all your woes can start from ignoring this area. New valves and guides by 60,000 miles or less - although my bike did not get them till 95,000 miles and my R100 has not had them yet and it has a similar mileage. A dropped valve could potentially knock out the whole engine if you are really unluckly.
Timing chain - replace when noisy - anything from 30 to 60,000 miles. Again, did mine at 95,000. It was cheap when I bought it.
All assuming you service the bike regularly, store it appropriately when not in use, and ride it with a little mechanical sympathy.
Don't go looking problems that are not there. And do not going having nightmares about what the previous owner might have done if you have no evidence to back it up.
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