2 wire thermostat wire want to be able to select fan only
Your furnace has fan terminals, thankfully
The good news is that your furnace has a fan (G) terminal and corresponding Rc and C terminals, found on the relay module depicted in your second photo.  The bad news is that the combustion controller is totally separate from the fan relay in your setup, so you'll need to wire this a bit differently from a standard furnace.  First off, your thermostat will need to be set up for two-transformer operation by moving a slider, removing a jumper link, or flipping a switch; the instructions should document how this is done.  Once that is done, you can wire yellow to Rc, green to G, and blue to C at each end, with the white wire going to W/W1 on the thermostat and one T terminal on the combustion controller, and the red wire going to Rh on the thermostat and the other T terminal on the combustion controller.
Note that while a 5-wire cable will get the job done here, it may be a better choice to put a fatter thermostat cable, such as an 8-wire or even a 10-wire, in now so that the wires are available for a future HVAC upgrade, such as fitting a heat pump.  Also note that if you're having trouble getting the heat to work, you may need to swap the red and white wires at the combustion control end (one of them is Rh and the other W, but your combustion controller doesn't differentiate between the two, while your thermostat just might).