" I hate to convert it to 12 volts."
There are a few legitimate reasons to convert a 6v tractor to 12v.....and the fact that it won't start is not a good reason.
If you can't remember the last time you replaced both battery cables, it's time to do that.
If that doesn't work, you can test the current draw on the starter. If you don't have a meter to do that & you are confident the battery is good, then check the voltage drop when you press the starter button.
Some one will be along soon to tell us what it should be...as I recall, voltage shouldn't drop less than 5.0v.