If sufficiently well-made and lapped-in, the conventional taper plug-cock is as good as any, and plenty of these appear among model locomotive designs. Designs for generic model engine fittings seem not to exist apart from a few samples in various text-books.
Cut the female taper using a D-bit type taper reamer made from silver-steel, on the same angular setting as turning the plugs. The better the reamer’s finish the better the mating surface inside the valve.
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The modern versions as used in domestic plumbing replaces the plug with a ball, held between two O-rings; one on the inlet, the other on the outlet sides. It is rotated by a tang working in a shallow slot, like a screwdriver and screw.
You can see how these are arranged by dissecting a scrap domestic-plumbing valve. Note that the diameter of the ball has to be significantly larger than the pipe bore for full flow. The slim-line domestic valves have quite small passages that must restrict the flow considerably. So a full-flow ball-valve may be a bit disproportionate but could be disguised as a globe-valve by machining the outside to a matching spherical form.
For the smaller model size, you could use a stainless-steel or phosphor-bronze ball, cross-drilled in the lathe with the ball gripped in a suitable collet resembling a pipe union.
Taper plug-cock bodies usually have a matching taper on the body. Make the body from two pieces of rod, silver-soldered together then machined.