I run a Salsa Cromoto.
It’s 5mm longer than the std Genesis fork but I can’t say its made a difference (for the worse).
I wouldn’t go longer as your defeating the point of having a non suspension corrected frame.
@ Andy … I though the Singular fork had more rake than a std 29er fork to compensate for the larger front wheel in the Hummingbird. I maybe wrong though.
Other options include Exotic, One One do a short carbon fork.
For greater versatility the Surly Pugsley fork (std spacing) is an option or if you have a bigger budget the Jones Unicrown fork is an option but that takes a 135mm spaced hub.