A simple machine that could be heated from a solar reflector or inside a
woodstove, and built at low cost would be a major step forward in energy
self sufficiency.

A double acting liner-alternator might sound simple, but really there is not
much additional complexity involved in using a crank and rotating shaft to
couple the engine up to a conventional rotary generator/alternator. A
double acting oscillating engine might be just as easy to produce, and
solves the problems of keeping the magnets away from the heat source.

Clifford Hall published such a flash steam engine in "Model Engineer" in the
late 1960s and has a patent for a boiler-less steam engine on similar lines.
(I'll try to find a link)

2 specific points.

1. Will the engine always self-start when the water first boils and flashes
to steam?
2. Will the piston always move and act as a pressure relief valve for the
flash-steam monotube boiler?

Expect about 5% overall thermal efficiciency, but if you are burning wood on
an 8kW or 10kW stove for home heating, 500W of power is certainly usefull
and better than no-power.

Use the volume of water in the home heating system as the hot-well, and the
radiators as condensers.