Saturn, you're right that he doesn't look a lot like Keats--they have a similar nose, though. I think Whishaw portrays more in the way of the Victorian interpretation of our poet (which makes me sigh and shake my head a little--when will we ever shake off Shelley's interpretation?!). After reading several interviews, I think Campion sees Keats as a tender, emotionally vulnerable, almost ethereal figure. Certainly, Keats contained those elements within his personality. Whishaw studied him more in-depth than Campion, however, when preparing for his role and he concedes in at least one interview that there was more to Keats (a darker side and a more "robust" side) that he found intriguing, but conflicted with Campion's vision, so he wasn't really able to utilize those aspects of Keats's character as readily.
Interestingly, Whishaw says in an interview that he does not see Keats as a particularly romantic lover--Keats's jealous nature and suspicion of women in general tended to poison any romantic tendencies.
As far as finding someone to be a double in looks for Keats, I still don't think that is *as* important as how his character is portrayed. Looks wise, I'd say Whishaw would be more of a double for Tom Keats than John. Perhaps they could have lightened his hair for the role, but as for the rest of it, I'm fine with Whishaw. . .as long as he's *not* a Shelleyan-Victorian pining and wilting flower. That would be hard to handle.