How to add a C wire to an old Lennox attic furnace GS810-105N

I bought a smart thermostat for my house in hopes of cutting some bills a little. I read up on them, I thought, thoroughly and figured I could handle the task. However, upon climbing up in the attic and pulling the cover, I discovered my unit does not have a labeled terminal panel and the wiring diagram is long gone.

I was hoping for some help with this as $300 is a lot to have sit on the shelf for 6 more months until I mister up the courage to climb back into the attic and stare at the unit confused for an hour again.

Like I said the wiring diagram was not included and there is no terminal board, just a mess of wire nuts but I did track them the best I could and made the list below. I'm not a professional so please excuse my ignorance when it comes to the industry vocabulary. I tried to pick up what I could from other forum posts but none of them were close enough for me to be sure of correlation.

From thermostat:
Red: ties to yellow wire with silver stripes that goes into silver box with coil thing (possibly internal tranformer? Is half way in the metal box and has two wires going into what almost resembles a roll of paper with two wires coming out inside the box)
Yellow: ties into red wire from transformer I believe?
Green: ties into post 5 on terminal bay
White: ties into wire from blower controller and blue wire that looks like it goes to the gas control

Other wires: white from transformer goes to terminal 4, second wire from terminal 4 ties to big blue wire from limit controller and silver box with paper coil (internal transformer?)

EDIT: while there are 5 wires at the moment (blue is not in use) I will be relocating the thermostat to the wall across the hall once I'm done renovating so I bought a roll of 18/5 to replace it with so I have all the wires I need at hand.

I'm trying to add an image but not having much luck so may try to in the comments to see if something is just hung up. In the image the red circled but is what I'm calling the internal tranformer, the blue is the terminal board and the yellow is the blower controller