The master cylinder casting is a 2006 or earlier cr/crfr (not x) sometimes ( but more rarely than sometimes) the japs have different size pistons for each brand, but I would buy a rebuild kit for the above models and it will most likely fit.
front brake master cylinder rebuild kit for 2007 EC300
My front brake wouldn't work below freezing without me heating the master with a hair dryer. Then it would work fine all day. Yes, a hair dryer HAHA. So I knew that's where the problem was.
I know you guys already know how to rebuild the masters, but I made a video of it during my rebuild (below). I used the AllBalls kit from RMATVMC for a 2005 Honda CRF450R. EXACT match for my NISSIN front brake master cylinder with an "11" stamped on the front of the cylinder. (I got online and looked for a master from another bike that matched mine, and it worked LOL). This kit doesn't come with a new piston (plunger?) but apparently some kits do, so double check if you think you need it. I didn't. Working in all temps now.