There was also a "calrod" heater that was available when the HT20D was a new tractor. FYI-- a "calrod" heater is kind of like the heating element on an electric stove or oven. It ran on 120V, so you would plug it in and keep the diesel engine warm so it would start with no drama. I saw one for sale on eBay within the last couple of weeks. The seller even had the engine shroud that it was installed in.
That rascal is still on eBay: