As a general rule: it's really dangerous to try to get into any situation thinking that you can change someone other than yourself.
For one, it is very hard to change your ways for someone else. It's hard enough to change for yourself. Clearly, lots of us on this site have changed our bodies over a long period of time; we've lost fat, gained muscle, improved our eating habits, improved our cardio fitness, or some combination of these. But for those of us who have been successful, how many have done it not from a desire to change themselves, but from a desire to please a new boyfriend? Obviously, some of our changes have been done at least in part to attract a guy's interest -- maybe even a specific guy's interest -- but when push comes to shove, we hit the gym instead of hitting the snooze button or the couch because we want the health and/or appearance that comes from putting in the time and the sweat that that entails.
For another, if the guy *does* change for you, there's a good chance that he'll grow to resent you for it. By placing requirements that he change to suit you on the relationship, you are effectively telling him "As you are now, you're not good enough for me. I deserve better than this, so shape up or I say goodbye." And that may be true -- you may honestly deserve better than what a guy is currently like -- but how many guys are going to want to hear that, make the change, and not grow angry over it?
Also, you're asking about changing someone early in a relationship. How well do you and the other guy even know each other by this point? What sort of assurance do you have that you're even compatible except for his fitness? When you're asking for major changes in someone, it's generally not a good idea to not have some sort of reason he should make them for you, rather than find someone who doesn't feel he needs to change.
The answers are totally different, of course, if you're in a long term relationship with a guy who was in shape when you got together and has since let himself go, but it sounds like you're talking about just becoming serious with a guy.
As for the smoking, yes, I admit it's a dealbreaker for me. But it's a dealbreaker for me from the start, so I don't even go on a single date with a guy if I know he's a smoker. I've told some otherwise interesting guys who asked me out that I don't date smokers, but if they decide to quit at some point they should let me know, but I wouldn't try to start a relationship with someone I knew I wasn't interested in for a specific reason. When you're trying to figure out if a guy is attractive enough for you, that's a little more difficult -- personality plays such a role in figuring out who we find attractive in real life, instead of just in photos on the web -- but if you can tell that you're always going to have one eye on the door checking out whoever just walked in to see if he's better than the guy you're with, I wouldn't recommend trying to make your guy hotter. I'd suggest finding someone else you're already happy enough with that staying with him as he is now is more appealing than trying to find someone better.