I'm not an authority on early M series engines,
But the info that I can offer is to be careful, as I'm pretty sure there are 'early' and 'late' 4Ms, which have different heads (Early has a round cam cover, late has the square 5M type cam cover), not sure if they have the same camshaft regardless of 'age'.
Sorry I'm no help at all, but I just thought I'd add this in case you didn't know.
Cheers
Wilbo
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