A very novel concept, and I am sure, even though you found nothing on your web search, someone has tried it sometime over the last 300 or so years. Maybe it was such a dismal failure, it was never recorded anywhere. But these things are here to challenge us, and as they say, many new inventions are not new at all, just rediscoveries of old technology that didn't work, brought into being by using new technology that would.
Fossil fuels produce their own heat in the combustion process, and is confined to the cylinder, where with a coolant jacket around the outside, temperatures can be controlled to very close tolerances, so the engine remains relatively cool.
What you would have to take into consideration is the expansion of metals and lubrication problems you would face in keeping the engine at such a high temperature, high enough to vapourise water instantly, without cooling the engine down, all the time it was running.
One way maybe to overcome it, and I do mean maybe, would be to have a very small flash steam boiler sitting on top of the engine, being fed by an engine driven water pump. The valving arrangements and their mechanical linkages would need to be worked out, and I am sure, if it was to be used with a fairly low pressure, slide valve engine, you could generate enough steam to run continuously from a very small flash boiler, and also the boiler could be insulated from the engine, to keep the engine at a much lower temperature.
Maybe a bit far away from your original concept, but one where you wouldn't have to take a degree in metallurgy to try out.