2- Is the engine liquid cooled?
--It's a steam engine; single-acting poppet valve variety; no
cooling channels required. If anything I want it to run hot.

This would have been useful to know up front. (grin) If it helps to
think in terms of the crankcase layout, first get the crank mocked up
(out of wood is still good - assumes you don't have one) . Then cut
out a large enuff hole in another piece of wood, split that in two to
get two half circles, and bolt it back to gether around the crank. Do
the same with your method of moving the valves and add that to the
'crankcase'. Now box in the 'crankcase'.

It occurs to me that steam engines leak past the piston seals quite
badly (also not a big deal most of the time). If you intend to run
the crank in oil and the crankcase is attached to the cylinder walls,
can you calculate how long the engine will run before seizing up from
the ensuing oil/water sludge?

Pete