Bill, forgot about the recirculating tank heaters.
We bought a couple at auto parts store but just google: engine recirculating tank heaters.
Depending were you live and your requirements there are different wattage of heaters. If you want to leave the tractor plugged in all the time on a timer then a 500W heater is probably good. If you just want to plug it in for 30 Min before start then a 1500W heater is better.