There are heat pump replacement units available, and they will work to heat until its about 37-38° outside and also cool in the summer. (In the south a lot of homes are equipped with heat pumps with axillary electric or gas furnaces for when it dips cooler)
Also, as mentioned above, some units you can add heat strips.
I also use a portable heater. Actually , I use a Heat Surge hybrid mini fireplace. It's vey efficient , safe, no stinky element type heat and the heat is soft and confortable. They are a bit pricey but totally worth it! It's also fun to have a mini fireplace
If it's a Colman they have a heat strip kit to install but you need to have an extra wire to run it. Mine has only 5 wires so I would have to pull a wire for it. The other option as mentioned is to replace the A/C with a heat pump as Miller Time pointed out. I just use the heater in the fireplace and a 1500 watt ceramic, with both on they will cook you out. If the A/C go's down then that would be a good time for a replacement to switch to a HP.