Here's a quick method that I've had good luck with even on tractors outside down in the 20's overnight.
I needed to feed as soon as I got off the schoolbus at 8 so the temp is real low and what sun was there was so low in the sky and late coming up so no warmth from it.
Go swipe the wife's hair dryer and pull the extension cord out of the shop to the tractor. Raise the hood and put the hair dryer in a fairly clean dry spot where the air will be directed at the head/intake manifold. On my Fords the battery is over the valve cover so if you prop the hairdryer right it will blow hot air over the head and intake, around the battery, and kinda recirculate under the hood back to the radiator area. Prop up the hair dryer securely (baling wire or wood blocks) then gently close the hood. I put it on there at 6:20 and at 8 am the tractor started up like it was a 90 degree summer day. Just be sure to take the hair dryer and other stuff out from under the hood before you start it. Takes maybe a minute to do and works like a champ.
PS. Don't let the wife know you do this. Clean and dust off the hairdryer and tiptoe back into the bathroom and put it wherever she keeps all her unnecessary junk that clutters the place up (all females do this
Put it back carefully enough and she probably won't even notice it's been gone.
You'll probably just want to swipe it for good at some point so you don't have to keep bringing it back. Get her a new one and tell her you thought she needed a nicer one so you bought her a gift. NOTE: NEVER do this on Valentine's Day or your ANNIVERSARY!!! That way you'll get brownie points for being a 'sweet guy' and the old hair dryer to heat up the tractor. Win-win scenario.
PPS-- If you don't have to get up at the butt-crack of dawn to drive griping miserable little ingrates on the schoolbus, you could always hook it up on one of those plug in timer things you can buy at Wally World for like $4.00. Depending on your temp give it about an hour or two.
PPPS-- Be careful if you haven't cleaned off your engine since it was new or have 3 inches of soybean fluff or corn chaff laying on top of everything under the hood. You might want to grab the old air nozzle and blow the crap out of there first before putting your hairdryer under there. Of course blowing the junk out of there is just a good idea anyway hairdryer or no...
Oh, and by the way, be REAL careful with those heat lamps. That USED to be my favorite method was to put one of those heat lamps in a clamp on brooder lamp rig and clamp it to shine on the manifold, head, injector pump, and the side of the battery. On a white cased battery it'll work, but on a black cased battery it can cause the plastic to soften and bubble up. Glad I caught it before it melted through the battery case. I never expected it to do that since the light was about a foot away or so and shining mainly on the head and manifold, with just the 'edge' of the light shining on the battery. BTW the hair dryer works a lot better and hasn't hurt the battery case, though I wouldn't put the hairdryer nozzle within six inches or so of the battery anyway.
Good luck! OL JR