Google 'Junkers Jumo 208 aircraft engines.' This opposed-piston diesel was used on the Hindenberg and in Junkers high-altitude recon planes in World War II.

In engines, there is little new under the sun.

More on the old Junkers Jumo 205

https://www.geocities.com/hjunkers/ju_jumo205_m1.htm

https://www.iet.auc.dk/sec2/junkers.htm

The Graf Spee ran on nine-cylinder two-stroke opposed-piston MAN diesel engines.

Fairbanks-Morse still makes opposed-piston diesels

https://www.fairbanksmorse.com/engine...rine_power.php

Opposed-piston engines must be two-stroke engines because they have no place for valves.

Two-stroke engines are generally very dirty (in terms of particulate and unburned hydrocarbons), but they are very tolerant of fuel quality. EPA emission regs have essentially killed the two-stroke engine in the US outside of locomotive and ship engines. Fairbanks-Morse engines are said to be able to run any fuel even sort of oily. The only thing that defeats a F-M is aviation gasoline. The Germans have even run opposed piston diesels on a oil-coal slurry. Being two-strokes, they can be very light for their power output.

Opposed-piston engines are seductive to nations with limited manufacturing capability because they have no high-precision valves. The Junkers Jumo had fourteen main bearings (easy to make) and no valves. A V-12 EMD two stroker has 48 valves and seven main bearings.

Never say never
 

I hadn't considered that possibility until you posted that it was not possible. So, just to be contrary: Move the ports closer to TDC, put an intake valve in one of the ports, put an exhaust valve in another. Ta-Daah! a valved, opposed piston, four stroke.
No? Why not?

Opposed piston engines have better surface area to displacement ratios, meaning that they lose less energy to coolant, for a given displacement.