You may have solved this problem already, but my '89 Spider has a tick like what you describe, I have done all the same things as you with checking and rechecking valve clearance, replacing exhaust gaskets and all. What I discovered is that one of my tappets over the valve shim does not fit the hole it goes into, I bought a new tappet and put it on and it only makes it worse. From what I understand is if the engine gets too hot, it can distort the tappet hole in the head and cause the tappet to stick a little and you get the ticking noise. On mine, when the engine is cold and I first start it up, the engine is quiet, but as it warms up the ticking gets intermittent until the engine is fully warm, then the ticking is very regular.
I figured this out by pulling the tappets out to check the shim sizes, seven of the tappets came right out with my little magnet, but the tappet on the exhaust valve on #3 would not come out with just the magnet. Like I said, I replaced the tappet and it got worse, so I am guessing short of pulling the head off and taking it apart and having that hole machined, there is really nothing to do but let it make noise, so far it has been several thousand miles and the sounds has not gotten any worse.
'86 GTV6 - No Name
'88 Verde w/no sunroof - Bella
'89 Spider Quad - Red
'86 Spider Quad - Silver - SOLD