Thermostat for outdoor boiler

Hi I am trying to set up outdoor wood boiler to heat my garage with water to air heat exchanger in plenum of old General Electric forced air furnace. Problem is thermostat only has 2 wires and only two to furnace (no control board) don't want or need gas portion to work. Just want thermostat to kick fan on when temp is low and off when warm. This furnace only will kick fan on with gas when it is warm and fan goes off when gas valve closes and unit cools. So do I need to somehow bypass internal furnace thermal switches? And if so how?

There isn't enough information to even begin to answer the question. A model number on the furnace would be the minimum starting point, which may (or may not) yield a web-searchable manual or schematic of it's controls.