Actually there is a Wilesco model of the D10 that was originally built with a throttle valve on the steam line. I believe it was a Sears contract model, so if you look out for one, and get lucky, you just might find what you are looking for.
Other than that, you might consider a Jensen 25 or 75 as a good choice for a reasonably priced smaller sized engine with a throttle valve on the steam line.
It is also possible, and often easier, to put a throttle valve (choke valve) on the exhaust of an engine, and frequently that only requires some silicone or similar tubing and an aquarium air valve to accomplish.
One other method for controlling the speed of engines without any throttle, is with the load you put on it. If you balance the work the engine has to do, with the speed you want it to run at, you can often enhance the entire experience, and make it work like a real engine, instead of just freewheeling.