“Mojo” is probably too recent for a WW1 game.
“Chutzpah” is probably not that applicable to goyim, and anyway should be reserved for the post-1948 extensions to the game system when you start doing Israeli pilots.
“Boldness”, “Audacity”, “Daring” or “Dash” are all about right, but not distinctively American.
If “guts” is not acceptable, other body parts associated with the quality in question are “spine”, “neck”, “bottom”, and “balls”. Spine is too complimentary; neck is too derogatory; bottom only makes sense in British English; and balls might be thought a trifle vulgar.
“Spunk” is probably the right kind of word, but liable to cause unwarranted childish amusement among British English speakers (which makes me recall that flight to the states when “Dancin’, Shaggin’ on the Boulevard” was a hit in the US, and playing on the in-flight movie screen — the flight attendants were bemused by the fact that the UK-born half of the passengers were turning puce in their attempts not to howl uncontrollably with laughter).
I would vote for “moxie”, which I think might be old enough for WW1, and sounds thoroughly American to me.
All the best,