My folks are into their 80's now and live in rural Maine. Their ONLY way of heating their home is with a wood stove - just a single wood stove. Up to this point, my folks have been able to tend the stove but because of health reasons, this situation can not continue much longer. Presently, there is no duct work. Unfortuneately, the house was built for a wood stove. My sisters and I are looking to have some type of propane gas heat system installed. What types of systems are available that can be retrofitted?
With no duct work already existing, there's nothing really to "retrofit". What you'll be looking at is a new install in an existing house, something that's ABSOLUTELY doable. In fact, a few years back I got an estimate to install a new HVAC system and run a new trunk up to the attic and the estimate was about 12k.
The only thing that wouldn't have been new in that case was the gas line into the basement.
Newer high efficiency furnaces can be had which don't need to be vented out the chimney but can simply be vented outside the basement wall and partway up the side of the house. Likewise running propane or natural gas is pretty straight forward.
The most complicated part will be running the ducting. The ductwork should be sized out by a pro - size matters, particularly for the air return, which if too small can burn out the furnace.
Nice to see a fellow Mainer on here. You should look at propane fueled stove, space heater which can be vented up the chimney with a proper double wall exhaust/ combustion air pipe. These heaters can also be direct vented right out a wall without using the chimney. These heaters are totally automatic,some have low oxygen shutoffs for safety, and come in a large variety of BTU sizes. If they are heating with only wood right now, then a propane heater is going to be like heaven to them, and much safer. What area do they live in? I may be able to refer you to a good supplier in your area.