Here are some photos of an engine I have had for years, I bought it from a guy in Boston who also was selling his collection of other scratch-built engines at the time, and had some interesting items. He described it as a "faux 4 cycle" engine. I have never known for sure, but a great phone conversation with Randy today prompted me to finally put it up here to see what he might have to say about it, as well as anyone who might have some commentary.
The two cylinders move in tandem, turning a set of gears on one side of the twin flywheels, which turns a two-lobed little camshaft below, that engages two pushrods that extend below the cylinder block, and engage two little spring-loaded rocker arms on the front. The 4th photo shows a pipe cleaner pointing to one of the lobes on the pushrod. I have run it on air once, it works fine but requires a minimum of about 20 PSI. Makes a nice sort of "blatt-blatt" sound when running. The whole thing is very well-made (and for its size, is really, really heavy). I have no idea what is going on inside there. I was thinking that maybe it is more of a faux hit-miss engine (?), but I know next to nothing about hit-miss engines.
Hopefully these photos are sufficient to show the relevant details, sorry if some are a bit blurry, my old camera seems to have developed some problems with reliable focusing!
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Sorry, "4th photo" should read "5th photo".