The exact reason for why yours is missing may never be known, but overheating is a common reason.
Some guys think removing the thermostat is going to help an overheating problem or make the engine run better. This is false.
You need to see if the lifters are solid or hydraulic. If they're solid, there will be valve lash on all the valves when the lifter is on the base circle, and the lifters will not compress if the rocker adjustment nut is tightened down enough to lift the valve slightly off its seat (also w/the lifter on the base circle). If they're hydraulic, the lifter will bleed down from the valve spring pressure, allowing the valve to close again. This bleeding down can take several minutes, so if you do this go have a coke and come back to see if the lifter has bled down or not.
Hydraulic lifters can be adjusted cold or hot. Factory settings were often 1 to 1-1/2 turns down from zero lash. Nowadays, we usually use less preload- on the order of 1/2 to 1 turn down from zero lash.
Most specs for solid lifters call for the engine being hot. The problem you may be facing if this is a solid lifter set-up, is you do not have any specs on the valve lash. You can start at 0.020" and go from there.
Too loose is harder on the valve train than too tight- until you are so tight that the valve doesn't rest on its seat long enough to rid itself of enough heat. When that happens, you risk burning the valve and eroding the seat.
If this is an original engine, there's a very good chance the cylinder head needs to be rebuilt. These engines were made to be run w/leaded fuel, if it's been driven extensively on lead free gas, the seats and valves can be worn out. The valve guides will always be worn, too. Along w/the valve stem seals being bad as well.