Top End Gone? No Clue

I am not the most mechanically inclined so forgive me if my questions are dumb. I have a 2002 cr250, the engine has been ported and polished through pro circuit. I was out riding on the track and heard a weird rattling noise and now I have no top end. What parts do I need to fix this and do I need anything special because the engine has been modified?
Also does anyone have any good recommendations as far as big bore kits go?
Thanks

The bike runs, it is a little hard to start, and has no power when I get to about half throttle. It definately has no powerband.