If I were to suspect my engine power has dropped a little, or if I detected a bit of rough running, I'd go for carb heat first. What else would you suggest checking/doing?
If it's rough running and carb heat (or alternate air) doesn't clear it, you can also try running on just the left or right magneto. If it's one spark plug that's gone bad, you now have all minus one cylinders running on two spark plugs (good combustion) and one cylinder on one spark plug only (less than optimal combustion). By switching the whole magneto off to which the bad spark plug is connected you've created a symmetrical situation that will - hopefully - clear the rough running. It will give you a slight performance degradation though, plus you've just robbed yourself of a backup system. So it might be a good idea to land at the nearest suitable airport and get things sorted.
Another thing to check is whether you've leaned properly. Over-leaning can cause rough running as well.
Further, you might want to play with the throttle, fuel selector and the fuel pump a bit. There's not a lot of failure scenarios in this area that lead to rough running, but in your average spamcan, that's about all the engine controls you have, so there's not much else you can do in-flight.