かっこいかった may be acceptable, but I'm pretty sure かっこよかった is more common. Otherwise it looks pretty good. イケメンだった or something like that may be a better word choice, but it's a well-formed sentence otherwise.
Your English sentence says "I think the concert began at 9 o'clock," but your Japanese sentence says 九時にコンサートをはじめたと思います. Do you see the difference?
[Edit] Well... I was a little late on this one. Oh well...