Cannondale has a head tube reducer to enable the fitting of a 1 1/8 headset and forks on their Headshock bikes. But that was back in those days when the are no oversize headset or fork. But now you can get Fox forks in 1.5 in. I have not seen anyone fit a modern Fox fork on a older model Cannondale, but since the Cannondale headtube is also 1.5 in I don't see why it couldn't be done.