Almost anything in the valvetrain can cause tappet type noise. Oil change is the quickest and easiest thing to do.
Could also be wear to cam lobes, blocked/poor oil circulation in the head, valve wear.
Do the oil change maybe some kind of flush (unless you've inspected the head and oil ports already and they are good).
If no good, then consider pulling the head for inspection of valvetrain against specs. (maybe bite the bullet and do an engine transplant, either reco engine or conversion).
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