As everyone has noted, these cabs are available but it takes time to find them stand alone. I ended up buying an entire 318 setup with blown engine just to get a very nice all metal and glass cozy cab to mount on my 322. I ended up selling many of the components I didn't need. It covered the cost of the entire kit and my travel. 16 hrs. Plus i kept a very nice and expensive cab.
Keeping the windows clear can be tough in an air cooled tractor. A fan helps. A small electric heater of 120watts (10amps) with a fan running will provide just enough heating to keep the windows clear. You will not be sweating to death but it can help. You will need to block off the pedastol vents so the engine doesn't suck what little heat you have out.
Some people have setup an auxilary alternator off the rear PTO to run larger heaters and extra lights. Maybe a couple deep cell batteries as rear ballast providing supplemental power to an electric heater for short (maybe 30min, you would need to do some math depending on the setup) periods of operation. I usually see several here in New England every year but they may be to far for you.
Note: amperage is limited to 20 or less on these tractors,so power is limited.