Assuming you have the FMJ magneto and it is properly timed internally.
1. Make sure you are on the compression stroke of #1 (Front Cylinder). Line the Fire mark up in the window at the bottom of the bell housing.
2. Mount the magneto to the governed shaft.
3. Remove the distributor cover from mag.
4. rotate the magneto backwards until the rotor approximately lines up with the 4-cyclinder raised bar (should be in the upper right corner).
5. Mount the assembly into the tractor so when the gear meshes you are still lined up on that mark.
6. Bolt the assembly down and then fine tune the mag angle so the rotor lines up with the mark. Tighten the mag
7. The wires from front to back of the engine are 1 upper left; 2 upper right; 4 lower right; 3 lower left.
The tractor should be properly time.