Well, there's an obvious once you see it physical choice for Buckaroo. Go to the movie's closing credits music number (findable on YouTube). At about 40 seconds in, after he rappels into the LA River concrete basin, there's a head and shoulders shot of Buckaroo...where it's clear that Matt (Doctor Who) Smith has the same head shape, similar hair, and satorical sense as Buckaroo. Heck, both Neil Gaiman and Paul Cornell agree with me on this.