Valve lash plays a very important role in engine performance,more so on the OHC engines.
But,normally over tight valve clearance only shows at running temp. Your misfire on a cold engine is different.
IS it a constant misfire or does it only happen when the engine is revved up?
if it only happens when the engine is cold and being revved,it is too lean on warm up.
You describe the plugs as being fouled so it sounds like the thermo is adjusted to a very rich at start up condition.
The length of the thermostat is very important hence the table i posted above.
Under the thermo is a little bolt on top of a piston,as the thermo pushes down it leans out the mixture. I would say the bolt is screwed down too far and needs to come one turn or until you get a leaner mix on warm up.

A slow thermo stat ( never actually seen one) would only make a hot engine run rich until it closed off the excess air.Check that you are getting flow through the hoses,no flow also causes rich running.
In the above post you can see how the thermo stat settings are arrived at.
You now need to check exactly when the thermo is closing,too soon or too late and adjust the mixture either way at the thermostat Only,not the pump idle mixture.
When running correctly,these engines can be started instantly from dead cold and will perform perfectly from cold without any thought of flat spots etc .