Maintaining a vintage 1950s-1960s Holland forced air gas furnace?

Hello everyone, after doing some Googling about my old furnace, it seems there are several members here who have an appreciation for these old reliable things, so I figured this would be a good place to ask about it. We moved into this house about two years ago. The house was built in 1925 and we're on the northern PA\NY border with notoriously cold winters. The furnace overall seems to work quite well. We replaced the gas valve the first winter we were here because it was leaking a bit of gas around the adjustment dial (which is apparently common on this type of valve?), and it was probably 30 years old. We have changed filters regularly using the cheapest\lightest ones we can get, because this thing likely wasn't intended to be used as an air purifier in the '50s. Is there a specific method to properly clean out a furnace like this? The first winter I used a tiny angled shop vac attachment to vac out the huge dust-encrusted squirrel cage fan as best as I could (with it still in place), but it seems like the heat exchanger and anything else inside it must be caked with dust too after all these years. I'm sure that has to decrease the performance and efficiency (not to mention pose a safety risk). We have a local "old timer" heating\plumbing guy who helped me adjust the valve after we replaced it, and he said "I wish I had one of these... it will run forever." He would probably be able to do the cleaning, but I wanted to ask some other opinions before calling up the only guy in town who will touch the thing. Here is a gallery with lots of pictures of it (I'll add more as I find them... I've taken pictures of it several times). I don't think I have pictures of all the various parts of it, but I can take pictures upon request. It has an amazingly cool mechanical fan control and we have it hooked up to a 3rd Gen Nest Thermostat which seems to work beautifully for a melding of '50s tech and modern "smart home" controls. https://ift.tt/3v6xzVh Any information, tips, schematics, pictures, filter suggestions or suggestions for improving efficiency, etc. would be much appreciated.

source https://www.inspectorsjournal.com/topic/18995-maintaining-a-vintage-1950s-1960s-holland-forced-air-gas-furnace/?do=findComment&comment=173536