Thanks for the links, guys.
I got my vertical single engine to run on Propane using two of my liquid fuel carburetor designs. I just removed the burner tip from my propane torch and connected it to the carburetor with a urethane tube. I had to fiddle with the propane torch control valve and the carb mixer, but I got the engine to run at least as well as it did on liquid fuel. Turns out you really don't need a demand valve as long as you don't leave the engine running unattended. I expect the carb jets and apertures need to be adjusted (enlarged from what I've read). I also ordered a butane adapter valve off ebay which will let me use fuel directly from a camping stove canister.
Chuck