That's it, really: can a bike like a Zaskar with a 1 and 1/8" threaded headset take a modern threadless of the same size instead? (I don't want to do this - I've just been reading up on headsets and can't find the answer..)
The headset just fits in the frame to whatever spec it machined to. Ahead came out at about the same time 1-1/8" came out way back turn of the 90's (91/92).
The problem is the fork needs a different style steerer (longer and smoother) and you need s different stem.
I upgraded my Trek back in the late 90s when the headset wore out had to buy a new steerer headset and stem, some Easton carbon bars caught my eye in the LBS too in the end cost me best part of £250, all of the parts are still in use today though even if they are now split between 3 bikes!
The Zaskar was one of the first production bikes released with an aheadset. I eventually fitted GT ahead forks to mine in place of the ahead rockshox MAG21