Hi Chris, yes it does use ratchets. It would make a neat model but I'm not sure it would be too stable. The real thing in the video would be pretty iffy with iron wheels on cobblestones in the rain, and no great braking system. Not too convenient to fire, either, with the furnace door at bottom front of boiler. It was a start, however, in the invention of self powered road vehicles.
There's an English 3 wheel car called the Reliant Robin. Much the same overall format as the Cugnot machine but it has a gas engine at the front and no whiskey barrel at the back. It's not too stable either - the TopGear show had fun with it turning at the bottom, going down a hill. Instant rollover- see link below. Anyway you could probably pick one up fairly cheap in the UK and build a Cugnot boiler on the front, convert the engine to steam, and remove the bodywork. Instant full scale Cugnot Reliant, with all the handling characteristics of the original Cugnot machine!