You buy a recirculating heater and problem solved. Those are great and in a car the heater works when you start it. It's great at way below zero. Here's what I mean:
https://www.walmart.com/ip/Kat-S-13150-1500-Watt-Aluminum-Circulating-Tank-Heater/144485009
Above is a 1500 watt Kat's recirculating heater. Put on any engine you can get a a flow from such as a heater hose and into a drain plug. This will circulate the water. Plan B for only one drain plug is from there to a lower radiator hose with that type of pump. Now, 1500 watts with pumping water will warm things up. For me, the magnetic stick on heater lost too much heat to the surrounding envronment and did little good if it was really cold, and that's when you need them.
A recirculating heater is nice in a vehicle or plow truck in a cold climate but for almost all of us with tractors, synthetic oil is so much easier. Also, read the thread from Adiredeck and how he put a freeze plug heater in his Kubota and inadvertently made a hole in the cylinder wall. What a pucker. He has posted in this thread I think.
http://www.tractorbynet.com/forums/...84-damaged-block-installing-block-heater.html
Just my two cents here and different things work for different people.