Brian, this is the principle of operation of the "rootes" diesel used in the "commer" truck and known as the TS3 engine, it was watercooled and had a blower to scavenge the spent gases when both inlet and exhaust ports were open, there are one or two examples of survivors on "U tube"...
A warm welcome to the forum John, from yet another telecom guy..Hampshire is a lovely spot and I'm not surprised by your interest in aero engines and planes, as Supermarine made aircraft there and isn't Farnborough there too? plenty of inspiration there!
The stainless cylinders could be heavily finned to increase their surface area for transfer of heat..also the two pistons and other reciprocating parts moving in the same direction will create a lot of inertia forces at the end of the stroke, probably would be better set up as a boxer engine...
These are the oil pumps favoured by Kirby Lauson on their lawnmower engines, seemed very dependable.
Though the delivery will be pulsating of course.
Stew.