RE: Best reliable, narrow, 40cc gas engine for WWI scale flight
I tend to agree with mrbigg. At 25 pounds and your wing loading (which doesn't take into consideration the parasitic drag from rigging), you should go with a 50cc or so. The ZDZ 50NG is an excellent engine, small for its displacement, very powerful and will turn as much as a 24 inch prop, allowing you to fly at much less than full power for a nicer in-flight sound and the more scale size prop. With either a side- or rear exhaust, it will fit your cowl nicely. RC Showcase sells them or try Chief Aircraft, I bought mine from them, with a Slimline Pitts muffler. They ain't cheap, but you get what you pay for.
Damn fine looking plane, nice to see such an example of a not-to-often modeled bird. Post some closeups.