When you go up in jet size you will be increasing the amount of fuel going to your engine, so you would need to increase the airflow through your engine to keep the air/fuel mixture correct. So, yes you would need an aftermarket filter and exhaust to flow more air. That's where the plug test comes in. You pull your plug out and look at the color of the end of the plug and you can tell if it needs more or less fuel. When the fuel mixture is right it will be a light brown or tan color.
The carb should have a drain hose and an overflow hose. The drain hose has a screw in the bottom of the bowl to close it off. If that is the one that is leaking, then the screw is either loose or damaged or the o-ring is gone off it. If it is leaking from the overflow hose, then your float is bad or your needle valve or seat is bad. I know you said that you replaced the float and needle valve, but if the overflow hose is leaking, it's still got to be the float or needle valve or seat because that's what cuts off the fuel from entering the bowl and yours is not cutting it off. But since you replaced everything, look and see if it's your drain hose leaking instead of the overflow hose.