To be sure you can use a test light, touch one lead from the test light to the frame of the tractor and the other lead to the Positive + terminal of the battery if it lights up it is negative ground if not touch the negative - terminal of the battery and if it lights it is positive ground.
One thing to check if it was converted to negative ground is that the switched the wire on the ignition coil.

Using your test light you need to turn the key to run position, touch one lead to the negative - terminal of the battery and on the ignition coil touch the Bat + threaded stud it should light and means it correct and do nothing. if not touch the Dist - threaded stud and see if it lights if it does move the wires on the ignition coil to where every wire on the Bat + threaded stud is moved to the dist - and every wire on the Dist - threaded stud is moved to the Bat +.
There is a good chance your charge gauge is backwards also , it swings in the direction the current runs and when it was reversed so did the needle on the gauge and in the case it does not mean it is discharging.

Most change to alt. is negative ground.