3D printers have tiny 12v 40w ceramic heaters only 6mm diameter and they are under $10! 1 or 2 of these in the right places should do the job, and you could turn them on before even starting the engine to make cold weather starting easier.
https://www.amazon.com/Generic-Ceramic-C ... mic+heater
https://www.amazon.com/Generic-Ceramic-C ... mic+heater