Hello Paul, never heard of ''Clapper' 'valve engine as such... but as far as I know, the Clapper valve is a form of ''Check Valve'' associated with ''Firefighting'' equiptment...may also be a fixture on steam locos...possible Check valve...must do some further ''Hokeing'' on this one. All the best for now, John.
My father built one years ago, it was single acting, the steam came out of a port at bottom of the cylinder it just had a ball on the cylinder head and a pin on the cylinder, when the piston rose it pushed up the ball to let steam in, i remember that it did run but not very well, i think it just had a too light flywheel.
Yes there are co2 engines for model aircraft that use the same design.
Paul I've seen a site with that sort of arrangement but using a car tyre (Schrader) valve, not really here so don't have my bookmarks, something College based in the US with lots of VERY simple arrangements - ring any bells?