It will depend on the stack height of the headset too. I’ve a bike here with a custom headtube for a Lefty. The head tube is 4.15 inches long or 105.41mm. That was designed around a Chris King headset, which has a stack height of 31.4mm. So 105.41+31.4=136.81.
As long as the overall height is less than the 137mm, it’ll be fine. The sizes above though, get away without using any spacers which makes for a tidier setup.
Edit: Are you still supposed to the deep-insert, Reset headsets on Nicolai frames? I know you can now get a tapered ‘Lefty for all’, this might be your best bet too, as I don’t know if you can get a 1 1/8″ to 1.5″ baseplate for the Reset?
I don’t think it would be wise to just have a 1 1/8th headtube on a new, expensive frame.