Hi Bike Roar please answer me as fast as you can 
Can a Lefty fork from a Cannondale F2 Caffeine 2009 be mounted on my Cannondale Trail SL1 2013 instead of her boring Recon Silver TK normal fork? Can any bike be transformed into a lefty? Thanks
Swap in a Lefty fork
Specific to the two bikes, we don’t think so. The F2 uses a Cannondale HeadShok Si, and while close in size (1.56"), it is not equal to the standard 1.5" of the Trail SL 1. Broadly speaking, not every bike can be transformed into a lefty. Recommend you look for a standard double-sided fork upgrade.