Have you got debaffled race exhausts? If not try swappping, sometimes the baffles break up and can block rather than rattle.
Maximum revs is normally controlled by the amount of friction or lack of it in the valve train.
So make sure that everything is moving freely, including valve stems in the guides, rockers in the ball joint created by the adjuster screw and the cam followers.
Also change rocker covers to make sure the breather isn't blocked causing too much crankcase pressure.
Otherwise it could be something 'silly'. Valves in the wrong way round, cam timing out or tappetts set on the wrong stroke.
Let us know how you get on.
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