rough running 1987 560SL

I have just purchased a 1987 560SL with 94k original miles on it. It clearly has an engine problem & I would like some advice on how to diagnose it.
The car, apparently, was running fine until one day, the previous owner started it up & it had a tapping sound & was running very roughly.
This problem has nothing to do with the timing chain, rails, etc. as they were just recently replaced (about 2 years & 10k miles ago)
Could this be a camshaft or rocker problem or ?
Thanks in advance for any suggestions on how to proceed with this.
Brian
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To quote you: camshaft or rocker problem (that wasn't fixed after the chain broke).
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