Right, so, IR'N GM. I got the following three GURPS soucebooks to run an adventure from.
Espionage. Not difficult.
Aztecs. Again, works for Who, or for a historical.
Horseclans. A series of pulp novels about post-apocalypse "Merihka" and the heroic psychic horse-riders lead by an immortal from the 1930s.
But I can grab an element or two. Weird people on horses... would be the Conquistadors. Or maybe I can use the villains, body-stealing immortal scientists.
Not the bloody psychic sabretooth tigers.