Maybe we should look for a 1000 cc Suzuki V-twin for the Metros...
The funny thing is, I have a perfectly good Suzuki 1.3L 4-cyl engine from the red Swift sitting in the back of a rusty truck in the bush. I also have WAY too many projects already
Years ago I was in a small-engine repair class and another student asked about a large, single-piston engine. The instructor said there would be too much lag time between cycles for the engine to be practical (or to possibly even stay running). He said you would basically get spurts of power during the combustion cycle. Sort of a BANG-pause-BANG-pause-BANG.
There are large, one cylinder engines in use out there. In glider school in Germany they used an old, average-sized agricultural tractor with a one cylinder diesel engine to set up their winch tow system and for other jobs around the airfield. You could easily hear the individual "pops" of ignition at normal operating RPM's and a slow chuga-chuga-chuga at idle.