I know the unit you used. You could keep the indoor unit and install an outdoor boiler using glycol that is more conventional than the unit you have. Also, you can replace the indoor unit with a direct vent furnace. Almost any manufacturer has direct vent furnaces now. I suggest you ask around the community for homeowners satisfied with local companies. It is important to use furnaces that are widely used in your area so you can have knowledgeable service and parts from a local supplier.
As an alternative, if there is room inside, you can place a direct vent boiler indoors and make hot water in a tank. This has advantages for further expansion into hot tubs and other heated water additions in the future.