This is going to sound like a total cheap shot, but here goes-
The honest to God truth is that the only muscles you should be working on in an effort to become a better puncher are those exact punching muscles. I.E. nothing helps you punch better than punching a bunch. Heavy bag work, sparring, shadow boxing, speed bag work, double bag work, and focus mitts are exactly what you should be doing if your intention is to improve your striking ability. Anybody who says their muscles can't get sore and/or stronger from all of the above hasn't been doing them with enough intensity.
If you want to take it a bit farther or even as far as KO power and explosiveness goes that is something that comes from the ground up. So you need to be working your legs for sure. Olympic style lifts can really improve your explosiveness because they put a heavy emphasis on the initial stages of the lift which include the driving power of the legs and an explosive opening of the hips. Study up on things like power snatches and you'll know what I'm talking about. Also- jumping rope is great because it improves your ability to stay on your toes for longer periods of time and being able to stay on your toes leads to improved speed and explosiveness because you're not trying to start a combination flat footed. Think of baseball players in the outfield being told to stay in a ready position and "on their toes" for reaction to a ball being hit their way.
Hope it helps