Well this may be a bit old fashioned but ill write it up any way's
we built insulated boxes round our engines out of tyvec and made a
little door in which to place a 60 watt light bulb.
when i had to start the mains once a month i would plug the light
bulbs in a few days ahead of time.
worked down to about -80 or so. when the mains started i would open
the far side of the box from the intake so they could breath till the
blocks warmed up. the gen sets i could keep warm on shore power, but
the thrust master mains could be a bitch.