Welcome Warcloud and for what my two cents is worth on this subject; I used them in the late sixties and early seventies when they were all the rage. They worked but the dip stick type would burn your oil in as little as 15 minutes if not removed. They are simply induction coils that heat as current is past through the element. They are available in several wattages from as low as 50 - 1,000 watts. If you insist on a mag heater go look at this site: http://www.padheaters.com/
Ole OB1's advice
consider the expense of a water jacket block heater. It will never cause a fire, if it shorts it will immediately blow a fuse or trip a breaker if 115 VAC plug in type. The coolant has a natural circulation because of the density of water Vs oil and that teamed up with a 0-30 synthetic oil will work in 40 below weather without engine wear during cold friction starts.