Free to fork the code?  Maybe.  Free to successfully fork the code?  In the absence of concise  and precise documentation of the bitcoin protocol and blockchain data structures this task is unnecessarily difficult and risky.  Unfortunately, the present "core" developers are doing nothing to change this situation.

Free to run the forked code?  Not on your life.  I downloaded and ran an XT node, but it was DDoS'd.  More accurately, my ISP was DDoS'd and my neighbors suffered collateral damage.  Human nature being what it is, this experience did not enamor me to any of the "small blockers".