Well I relocated the sensing bulb under the heater, no change. For testing purposes I heated the sensing bulb lightly with a propane torch's heat (not heating directly but letting the heat rise up to the bulb), and sure enough it kicked off. It was hot enough by then to be uncomfortable to touch, but tolerable, however way too hot to be of any use.
Going to try to locate a new gas valve with thermostat, it appears that it can be done, saw an example online of a different but comparable valve being used with a standard wired thermostat.
