BVP Bill-
The Wallas and Dickinson Propane Fireplace are great solutions to the heat issue on your 19, the first costing about $2000 depending on installation costs, and the second aboaut $600 plus installation and propane lines and tank. Slightly less expensive would be the Force 10 Cozy Cabin Propane Heater at about $400 plus as above. All these heaters are externally vented through exhaust tubes.
Any heater installation on a closed in boat should also include a carbon monoxide alarm for safety reasons, which can cost as little as $50, or up to $100 or so for more sosphisticated ones. Propane powered stoves should also be accompanied by a propane detector for leaks and flame outs.
Since your 19 is not totally sealed (even if it has canvas over the open back), you might want to consider a portable heater if the cost of the more permanent ones seems out of reach. Such a heater does not have an exhaust tube and is allowed to vent into the cabin.
The dangers from carbon monoxide, propane, and gasoline fumes are real, and I certainly don't want to be the one to recommend anything unsafe. But if the closure of your canvas in the rear of the cabin is not all that tight, a much less expensive portable heater like the Mr. Buddy heater will probably work fine providing you use it with a carbon monoxide detector and a propane detector. This would get the heater cost down to something in the $100 range plus maybe another $100 or so for the two detectors.
To my knowledge these heaters are used routinely in tents and other applications where similar ventilation conditions exist, and should be fine in your 19 with the detectors. Their combustion is catalytic, and the amount of carbon monoxide is quite minimal.
If I were to use such a heater, I'd be insistent on fabricating a solid mount for it, so as to make turning it over impossible.
I'd also want to put in quite a few hours of daytime use with the set-up before trying to sleep overnight with it, partially just to prove it's functionality during the day,aand then partially so that I could set my mind at ease and be able to sleep at night!
Worth a thought at least!
Joe.