While it is easy for us to pick out why you have a problem it is often hard for us to tell you the real problem since we can not see the tractor.
With the way I have seen some people wire these tractors I am only guessing. That is why I give you a diagram. You could be using the amp meter; key switch; or even a terminal block to connect wires. All will give differant problems and will change my answer.
Lets assume we are using all the stock parts and go from there. You can see there is a wire (yellow if orginal) that runs from the starter relay to the amp gauge. This will give the amp gauge power 24/7 and you said no power with key off. Hence we know you have it wired wrong.
How it is done is the question.
You could have the wire from the relay direct to the terminal block A side and the amp gauge voltage regulator to terminal block B side. This will cause all power to run threw the key switch.
Another way of saying this is......
Look at the terminal block in the picture. The top bolt is hot 24/7 It has one side of the key; power from the battery threw the amp gauge; and the voltage regulator B post.
The other side of the terminal block only has power with the key on. It has the key switch and coil only. So the only power that is switched by the key is the coil.