Richard the reason I asked about your tractor is if you really wanted to help it start in the cold cold, you might consider a block heater, wick isn't hard to install. But does take a bit of work. A cheap one (400w) are about 35 bucks and are about as worthless as that pan heater you bought. A good 1000w runs about 120 bucks but works quite well. Installation requires draining the coolant from the block, removing a freeze plug, installing the heater unit and refilling. When it's about 20 degrees outside you can plug it in for about a half hour before starting and the tractor should spin right over. It doesn't make the block toasty hot but does warm the engine enough to make a huge diffrence.

I thought I'd mention it, but you might get by with a better battery. The glow plugs pull hard...