Assuming it is valvetrain noise, if you can pull the valve cover and
start the engine at idle, you can tell which one is the offender by
placing the palm or heel of your hand on each valve or rocker or
whatever (not sure what the valve gear is like on that). The one making
the noise will be apparent by the amount of shock you feel. From there
it's a matter of finding out why there is slack in the valvetrain.

Looks like this design uses an OHC, with cam lobes bearing against a
follower. Lifter is at one end of follower, valve at the other.

Could be:

Bad lifter (but you've replaced them all.
Bad cam lobe - worn past the range that the lifter can adjust for.  In
this case the cam &  cam follower would show distress.
Worn valve tip - mushroomed usually.
Missing valve spring shim or steel insert between aluminum head and
spring base.
Damaged valve keeper?
bad cam follower pivot bearing

Is there anything else in this valvetrain besides lifter-folower-valve?
Hard to tell from the illustrations I'm finding.

If you find the offending valve/follower/cam lobe but can find nothing
wrong with it, then you may be able to shim the lifter up enough to
quiet it down. Usually the lifters sit in blind holes in the head. You'd
remove the lifter, and drop a washer or something similar into the hole,
then reassemble.  Note that this is a last resort.