Anything from the sixties should've had a chassis refurbishment by now, if it hasn't expect weaves and wobbles above 70mph. There used to be a great war between proponents of pre-unit and unit twins but when the factory went bust that died down under the influx of huge numbers of Jap invaders. When heavily tuned the whole motor disintegrates, everything from crank to top end to the transmission. Stock bikes have chronic problems from the primary drive, top end and the insidious nature of the vibration, which can take out just about anything. Bikes on the market vary from those that've been neatly rebuilt, using improved modern parts, to those that are just waiting to blow up but have a nice enough appearance.
Engines rattle all the time but knocks indicate some bearings on the way out.
Parts are still available for the engine whilst chassis bits can be mixed and matched to the owner's taste. In fact, some of the best bargains are found amongst the mild customs. Ignore the fanatics who demand originality, the Triumph twin, with its gutsy engine and agile chassis, is all about riding and excess fun. Decently rebuilt, set up to maximise the sub 5000 rev torque, they are even reliable. For those not obsessed by speed, Triumph twins represent the best deal on old British twins.