I have been a fan of one and two cylinder engines for many years. I grew up seeing these old timers putt-putting away at the county fair. Stationary engines still have a surprisingly large hobbyist following in the U.S. and Australia. Steam engines dominated from the 1860s to 1890s. Then came several different styles of one and two cylinder gas or diesel engines. They were eventually supplanted by higher compression (Briggs and Stratton style) high RPM gasoline engines. Because of their simplicity, low compression/low RPM engines still have considerable utility for grid-down survival use. They were common on most American farms until rural electrification programs got into full swing and as high compression engines came into vogue. Here in the U.S., they stopped making low compression stationary engines in the 1930s. But I was surprised to read that they are still making low RPM Lister-type engines in India. See: http://www.boingboing.net/2005/10/03/listers_and_other_ol.html. (One thing about the Third World mentality--they never discard a useful set of tooling! Perhaps we should learn something from that...)
If you are worried about a long term TEOTWAWKI, I consider these "appropriate technology" for retreats. They are low RPM, most have "bomb-proof" cast iron cylinders, and they are easy to maintain and re-build. With a good size flywheel they can be used to run generators for battery bank charging. A small steam engine would work, but they are a bit more tricky to operate, and generally require more maintenance.