Use an automotive "trouble light". That's a light with a normal incandescent bulb, long chord, with a hook on top of the fixture. Use one that has a metal shield. The night before a ride, turn it on and place it as close as you can get to the bottom end of the engine, but also close to the cylinders. Keep it away from wires & hoses (no problem on these modern bikes). Put a big tarp over the whole bike to retain heat. You'll be surprised at how much easier it is to start the bike the next morning. I've cranked my Shovelhead below 20°F using this method. The better you trap air with the tarp, the better this works. A blanket over the tarp helps, too.

One of my sons used to drive a "beater" diesel engine car. We used this method to make sure he got to school on time during the winter.

I got this idea from my Daddy (RIP). When I was young, we used a well for water. It had an above-ground pump and tank. He built a small shed around it. One winter, it froze up. So he installed a light fixture, and insulated the shed. He kept a low-wattage bulb in the fixture. During the winter, he left the light on 24/7. The pump/tank
never froze up again.

Tom

My 99 EFI Roadking will refuse to start below 45 degrees unless I used a lightbulb and cover.