I am an Electronics Technician for the military. So by no means am I an expert on the electrical wiring of a motorcycle. However, It sounds to me like this is your problem:
If you connect a volt meter or test light from the negative post to the frame and you get 12 volts, that means two things. The first thing is that the positive post of the battery is electrically connected to your frame when your switch is on. The second thing is that the negative post of the battery is not hooked to your frame or you would have shorted the battery every time you turn on the switch and you would get a lot of heat from the battery and probably a lot of other problems in your electrical system. The frame must be grounded and the positive terminal must be removed from the frame. The point where all this is happening is where you are going to have to use your investigative skills.
+1.....in other words...sounds like your battery is backwards. You can get identically sized batteries that fit your Sporty's frame that have the + and - posts on opposite sides.
With the ignition switch off, take an ohm meter and measure from the engine case to the plus battery post. I should be open. If not, swap posts if your cables allow it or get the correct battery.