I use a 200 watt magnetic block heater I got for under $20. It has a good strong magnet so I just leave it stuck to the block all the time. It doesn't get below 20 deg. here often but the beast always starts right up.
I hope it actually works like that for you Dave!
However I have my doubts based on my experience with magnetic type heaters they may have their place but I have not found one on a tractor. I had a 200w once and it didn't do a dang thing stuck to my oil pan which is a flat plate and should have been ideal for it to work heating the oil and it did absolutely squat.
... havnt tried it yet ... note to self though ... thanks for sharing your experiance; ... I wonder if some insulation over top would of the heater and blanket over the engine would help direct the heat into the tractor engine ...