This may sound silly, but aquarium air valves work rather well for this sort of thing. They are inexpensive and are designed to throttle air. The fit of the stem in the valve body is sometimes a little sloppy, so don't bother with this option if you are a perfectionist! I also just looked at some plastic valves on my granddaughter's aquarium, and they fit and work quite nicely, but don't add anything in the way of "style".
On the other hand, the scale globe valves that have already been mentioned would greatly enhance the look of the engine if you don't mind doing a little machine work on them.
McMaster Carr offers some small needle valves that would probably work well, but they would not be scale either.
One more option would be to try a valve made for the water line feeding the ice maker / water cooler in a refrigerator. They also are inexpensive, but may not work well for throttling. They definitely would not look right, though.
Just some thoughts!
