I’m surprised that the reproduction parts guys haven’t come up with a bypass thermostat yet. What does it take to entice them to make a particular part?
Tooling up to produce a complex part to service the few remaining medium and heavy duty truck engines that mostly went out of service 40 years ago doesn't make good nonsense, investment-wise. If someone did choose to make a limited run of that type of thermostat, each one would probably cost a hundred dollars or more just to break even on production costs.
Jerry