i wanted to know who makes a good bore kit and high compression pistons for a 250ex, and what cc`s would it make it

 

There's not a whole lot for that engine out there. From what I've seen, Powroll is about the only company who has really put a focus on that engine beyond a pipe and filter. I think their bigbore puts it near a true 250.
The cylinder, and cases, cant take a very large oversize piston or sleeve due to the engine design.