Mine has just a hair under 1.25" and its gonna be very very tight, clearance wise.
I may have to ditch the 5/8" shaft collar and find a narrower one as right now it looks like my seatpost tube is JUST going to nick it a hair.
Here is my shifter kit mocked up on the 67cc HT Im using:
I should have it on my bike tommorrow evening. Wont be running yet, but I'll at least have the engine bolted in.