Most engine people have had an engine with rusted, damaged or missing valves that needed replacement. A slow-running show engine can get by with a lot of problems, but when a part is missing or unrepairable, the engine is shut down. If you are lucky and have a common engine you can order a new valve, but the price will be $20 to $35 each. If you have something rare, the price is "contact us for details." You can bet it'll be over $35. But there is another way to replace valves for engines of this type.
At the next swap meet you go to, just look in the junk parts boxes. You should be able to get a handful of old, unidentified valves for a quarter apiece or so. I look for valves with stems around 3/8-inch, which is a good size for most of these engines.
Photo 1 shows an original Nelson Bros. valve with a burnt spot at the arrow. The other valves came from unknown engines. The one on the left has a stem that is too small. The one on the right is too large. The last one, as Goldilocks said, is just right.
"Just right" means the stem is the same diameter as the original. The head is too large and the stem is too long but those problems can be fixed. If you use the original valve as a pattern and trace the size of the new valve with a Sharpie as shown in Photo 2, you'll see how much material has to be taken off.
Photo 3 shows the valve chucked up in an electric drill and being ground down on a grinding wheel. A lathe is not needed, as exact precision is not necessary. Just grind until the entire Sharpie mark is gone and you will have the correct diameter as Photo 4 shows. Of course the rim is too thick and there isn't enough face area. Just hold the drill at a 45-degree angle and grind off enough for a proper face angle. It is better to use a fine wheel than the coarse one I used, but it worked.
The stem is still too long and is hardened. Mark the proper length and heat that spot red hot as in Photo 5. Cool slowly to anneal. Now it can be cut with a hacksaw. Drill a 1/8-inch hole in the tip for the cotter pin and your valve is finished.
A normal valve job with plenty of grinding compound will lap everything in and you will have good compression for a good-running engine - without spending the price of a dinner out for your whole family (Photo 6).
If your engine's valves are totally missing, you can determine the proper stem size by slipping bolts of differing sizes in the valve guides. Head diameter would be very slightly larger than the diameter of the valve seat in the head. Stem length would be just long enough to allow for a spring and keeper. Looking at other, similar engines could indicate how far a valve should stick out.
The last photo shows the Nelson Bros. Sattley head with old repairs. Note the valve guide was brazed back on and the rocker arm stud was broken off; it has a homemade valve and it still runs. If you mess it up, just try again. It only costs pennies and a few minutes of your time.
Contact engine enthusiast John Hamilton at: 910 W. Marvin Ave., Waxahachie, Texas 75165; firstname.lastname@example.org