Somehow, I prematurely posted. Continuing:
In the event that head /valve problems are diagnosed, have shopped online to see what a replacement head costs.
Several sources list Taiwan replacement heads, said to be CAST IRON ????
Is that an erroneous listing? Or, are there actually cast iron replacements for the original aluminum heads?

If I plug the jet valve holes, when is going to be the effect on smog test? On performance? On mileage? Overheating Or, what might be some other effect, if any?

On the flats, near the ocean, temperatures are normal for routine grocery shopping or other 5-15 mile trips. Thermostat was replaced about ten years ago. Have not tested the current one. Drove from 40, 000 to about 120, 000 (?) with a stuck or defective thermostat. With only VW experience, plus some Ford Escort misadventure ( bad module, a common problem with Escorts, before Ford began buying/ jointed with Mazda, which were much better, from experience until my sister crashed hers), I drove the D-50 and wondered why it ran so cool. A mechanic said " count your blessings". Maybe that contributed to the dirty condition noted in Tijuana when the Mikuni carb was cleaned at Carburadores Rodriguez, for a very low price by US comparison ( if a shop can be found yet who will do a cleaning and know Mikuni/Solex and are willing) .
The powerful solvents used in Tijuana are hazardous to the workers, and probably banned in California.