I also have/fly an older 172 with the O-300, great engine and plane that I have owned for 30+ years.
My other toy (plane) with the Rotax has a barely usable heater. Mine only has muffler type heat and in the very cold winters we have up here (northern MN) it is almost non-existent heat. I have been meaning to put in a small heater core that uses the coolant for better heating, but just never seem to remember doing it until it gets too cold in the hangar to do the work.
Definitely install a couple of the 'Earls' type heat exchangers if you are wanting to fly in the cold.