I lived aboard in NE for many years. First on an OI 41 on which I had a dickenson diesel fired heater. I believe it was about 10,000 btus. It served well for its cost and supplemented with electric. You could only run it when on board.
When I got a larger boat I had a 45,000 btu Webasto diesel fired forced hot water to air 3 zone system put in. This worked great. With 360 gallons of diesel on the Irwin it was as comfortable as any home with a much better view! The hot water lines were below the floor boards which kept the bilge dry and the sole warm. The down side is that this is very expensive and will only justify if you plan on living in the North on this boat for many years.
I would recommend going with the existing system for year one. Have your electrical system completely checked for safety and supplement with electric. You can re-evaluate next summer.