That is what we are going to do. Plumb in a heater core with a small electric fan on it, at least help keep the chill off.
Princess Auto should have something.
What I did was obtain a school bus rear heater from a scrap yard.
They are about 6" X 6" square and made for an excellent tractor heater.
Inlet and outlet was 1/2" so I used regular 1/2" copper pipe for straight runs and heater hose to link into engine lines.
Has worked great for over 5 years for me.
Trial and error proved that best mounting location was near cab roof, upper right hand corner thereby blowing heat down towards feet and if necessary a deflector can blow some hot ait onto wind screen.
I did modify the fan by installing a 4" 12 volt computer type muffin fan which is far more efficient and draws a mere 1 amp or less. Also much more compact a fan. Also much quieter.
I use my tractor all winter in our real cold Canadien climate and am 'nice and cozy'.
Think I paid the scrapper about $20. for the heater, and yes the same unit is available from most auto parts for about $200. c/w the noisy blower that draws enough amps to run a battery flat in about 5 mins. (think the brand is Carter).
Hint;
Even though you have a nice and cozy cab now, still wear warm clothing.
I once had a problem that required me to walk back for a piece of repair hardware and near froze my a-- off! ( I now carry about 20lbs of spare pins bolts tools etc-just in case)
Last mod was to install a coat hook for that 'emergency' parka.