My Great Planes PT-17 weighs 13 lb, and it is recommended to be flown with up to a 2 or 4 stroke stroke 1.20 (although mine is electric). A Ziroli Stearman would presumably weigh only four pounds more, so a 1.40 should be doable, at least on the low end. Military Stearmans aren't meant to fly like a Pitts anyway.

What 1.40 engine do you have?

JMHO.

Rick