He started this company in our basement: www.conwin.com
He and I were both Ham Radio operators when I was a teenager. So, we're both pretty comfortable working with electronics.
Things like relays and electrolytic capacitors and bad solder joints we'll fix. More complex stuff we'll toss.
The boards in the Payne/Day&Night furnaces back in the 70's had those boards with the "cube" relays on them. Easy fixes.
Actually, boards can be quite fascinating. Follow the traces on the back of them and you can learn a lot. When you follow a trace to a "chip", you've hit the end of the road, but there's lots you CAN trace that will serve you well down the road.
I found a source for rebuilt controls when I was in a spot looking for a Honeywell 7100f discharge air controller for modulating screws. No longer made and none in stock anywhere. I found this company and they sent me one with a core charge to return the bad controller. I noticed on their site that they had ignition controls and many others. I am not plugging them, but the name is acme controls. They took care of me, but I probably won't use them for anything that is still available new.