A 1.08 c.i. engine equates to about 17cc's........obviously a 77cc engine would be a little much. Usually the manufacturers recommended engine sizes are only a guideline. I usually figure out what the weight of the recommended engine is and start there....I go with the highest displacement for the given weight range....unless you're going to have to add a bunch of nose wieght anyway....then I'll go bigger.....much better to add engine displacement than lead....you can always throttle back.

A 77cc radial would fit well into a 50cc sized warbird. Power would be comparable but it would weigh a bit more. The Top Flight 1/5 scale Zero is a 50cc sized aircraft so if that is what it is going in...you should be fine. If they recommend a .90-1.08.....these are way too small for that sized aircraft.

CB