Justin have a similar look to Goku, and have some experience in acting, I understand that in honor to the culture that create the manga and anime, he could have been choose an Asian person, but no asian have a look similar to Goku, Chatwin have some apearence, only maybe need to get fit and bulk a little.
Not easy to find an asian that know english and can act, an Asian guy that can not act could ruin the movie, is just about find an actor that can play the part, I am sure they prove many guys before decide to go with Chatwin, and i am sure that many asian guys were competing for the rol, now lets give Chatwin a chance, mostly because is nothing left to do now, is not in our power to change nothing.
They at least give Chung (Chi chi), Joo (Yamcha) and Chow (Roshi) the rols so Fox is respecting the Asian culture, so is not like they are disrespecting asian people.