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Gotcha.
So that has a Cannondale headshok head tube. It looks like it has the Headshok headset in it which takes 1.56" steerers.
If you want to run 1.1/8th, tapered or 1.5" forks you need adaptors or a new headset.
Which fork? The easiest way of getting this running is to fit a Cannondale Headshok or a Lefty. Both use the 1.56" for steerer diameter.
If you go for a lefty you have a choice because they do 1.526" straight steerers for 1.56" headshok stems AND 1.56" tapering to 1.5" for Supermax 1.5" stems.
Clear. As. Mud.
Which frame / fork? I can confirm.