I've also seen magnet block heaters.. probably similar to that battery heater.
In the winter, i used to leave a trickle charger at around 50-100ma on my yanmar, if I new I was going to be using it. Also, as I didn't have a block or dipstick heater, sometimes I would leave a shrouded light ( trouble light? ) on, hanging off the hood or something similar. Always worked great.
We do this with some fruit trees, and also the water pump sometimes when it is going to hard freeze, with alot of wind. Just make a windbreak, and the light does the rest.
Soundguy