I"ll add a few more things. Spark plugs should be a non resistor plug. If you are not working the tractor continuously or putting it under hard loads then you may want to move up to a hotter spark plug but stay with non resistor plugs.
Champion recommended originally from what I have looked up a D16 spark plug. Most of their catalogs now recommend a D18 as the standard spark plug. If you run it with light loads or if you have an oil burner you may want to go a D21 Champion which is hotter than the D18.
Autolite plug is a 3116. Some will say a 386 is recommended but it is a resistor plug. Autolite doesn"t have many choices for a non resistor plug. At one time they made a 3118 which is hotter than the 3116 but in my latest search I did not find those anymore.
AC Delco offers a C85S which is non resistor but the C88L is resistor.