I'm not sure this magnetic heater will help much. After leaving it on my tractor for an hour, the oil pan is just warm and only in the spot right around the heater, and I think the max temp on this one is 150-degrees. They make a bigger version that may work better.
I can't imagine that it really "heats" up the engine that much anyway. There's so much cast iron, water, oil, void space, etc. in an engine block, that it must dissipate rather quickly and easily. Although probably not a real safe alternative, I would think a real heavy blanket over the tractor with a space heater running under or a trouble light on under it would heat things up better.
One of the things I've noticed with my diesel tractor is it will start fine, but the hydraulics take FOREVER to stop whining. Cubs are exactly the same way. Hydraulic fluid has to look like molassas at those temps.
1951 Farmall Cub, Cub Cadets 102, 104, 1811, 1864, Simplicity Legacy XL 4x4 Diesel with FEL, 60" mower, 50" Tiller