"I've told you before, Ron, keep your nose out if you like it the shape it is. Can't see why you would, but—"
From the context, this is colloquial speech, not formal speech. Artistic license and regional dialects or character-driven speech patterns may differ from strict grammar rules in this type of writing.
Even so, this example seems like a fine (if informal) literary construction to me. It is roughly equivalent to:
Keep your nose out [of my business / things that don't concern you] if you like...[your nose] the shape it is [currently in].
Here, "it" is used twice as a pronoun that refers to Ron's nose. Given the context, and the somewhat clipped speech pattern of the character being quoted, various words have been elided by the author in a way that still seems clear to me while invoking various common idioms about "being nosey" and related risks.
In this case, the apparent meaning is that Ron is intruding into something Fred thinks is none of Ron's business (e.g. sticking his nose in). Fred is either making a physical threat (i.e. Ron may get punched in the nose) or using a somewhat witty metaphor to imply that such behavior is risky for Ron.
In the second sentence:
"Can't see why you would, but—"
is probably a not-so-subtle put-down. Without additional context it's hard to be sure, but it reads as if Fred is calling Ron's nose ugly. Another way to think about this colloquial sentence pair might be to restructure it to see how the insult follows the threat:
Mind your own business if you like the current shape of your nose. I don't know why you would like it since it's so ugly, but it's probably still good advice to keep your nose out of other people's business if you don't want to get it broken.
Taken together, this is most likely the semantic meaning of the two sentences you posted. However, it loses a lot of the flavor of the original prose. Presumably, the original construction is more consistent with the rest of the material, and certainly seems more colorful.