Depends what you mean by "critical".
The engine will still run ok-ish with really worn rockers, but they will be very noisy and difficult to adjust.
The rule of thumb when rebuilding a motor is to use a new rocker shaft at least. If you can run your fingernail along the shaft and feel lots of ridges, it's had it and needs replacement.
They're pretty cheap so for the peace of mind and quieter valve gear, I'd get a new rocker shaft. Then dissemble all the other rocker sets you have and make up a set with the least wear on the new shaft - nice and quiet!
cheers
Jacob
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'72 Clubman Van - 1022cc, 295 head, 731 cam - Daily Driver

'69 Morris 1100 S - Dinged by a bus, in shed under repair
'64 Morris 1100 - Early 1100, long term project
