I´ve got an 1x1 from a couple of years back when they were recommended 80 mm of travel if you went with a suspension fork. I currently ride it, and have been for a couple of months, with a 100 mm fork and it works but not perfect and i´m switching back to the stiff fork as soon as i get around to it. I´d say if you got the older model that i got it will work with either 80 or 100 mm travel but i wouldn´t even try 120 mm. If you got the newer 1x1 that is said to work best with 100 mm forks i´d say go for 120 and see what happens.
I've got the 2012 1x1 frame with a 100mm fork, and it seems a little raked out compared to riding with the rigid fork...I would not suggest going 120, but that's just me. It depends on what you want and what you ride, though.
I have the 2014 1x1 and for a while had a FOX TALUS 80-120mm fork on it. I ran it 100 most of the time, but for chunky stuff 120 was awesome. Steering wasn't as bad as I expected it to be. Handled like an all mountain bike.