Battery blankets aren't that expensive & will help, assuming you can get it to fit. The ones that just have a little pad that fits under the battery seem to work. We have them on our stand-by generators. 

However, also at the same time, I'd add a good float charger for the battery. I think the Noco 2 amp chargers for "on-board" mounting are the best. 

Neither one of them draws much power and the input 120V can be hooked together somehow. 

A nice warmed fully charged battery should whirl a gas engine fast enough to fire, especially w/fuel injection.

PS....don't forget to UNplug the 120v source before driving away!