Some of the names of boilers (I think they all have them) that you might want to research are:
Central Boiler, Taylor, Mahoning, Wood Master, Heatsource1, and Heatmor

I have an indoor boiler that is not made anymore, but has been operating for many years, and is simple and heats 2100 square feet without much problem down to -10 degrees. It has a 9 gallon water jacket around it.

Look up under Country Projects - Indoor/Repairs and see Hot-water heating with Wood/Gas to see a thread on my setup if interested.