My 850 was temperamental if not under cover, with one set of ignition parts it had years ago, and they might have been Napa - Echlin too as I know I did that tune up as well. I did a tune up with parts from Napa (Echlin) within 3-5 years ago, but anytime its outside, I take a green tarp, (little heavier one from tsc) and use a few bungee cords to secure the tarp, keeping the majority of the tractor covered up. Tarp is not to the ground though. I use one just a bit small, to let air flow under, so it breathes, rarely do I have to clean points if it sits. I ran it last week and it sat since mid may, and started like usual.
My old K10 plow truck, an '80, same year as yours, was funny about those high humidity, damp days when its cooler, but not freezing. It would start and run, but not idle without stalling, it would hesitate and just not run right, but if I could run it for just a bit, seemingly get some heat going, then shut it off for 15 minutes, it ran like it should for as long as you needed it. Funny how it would do this every time and it has HEI. At the time when it was still road worthy, (early 90's) I had done just about a complete frame up on a '78 vette, with a nicely done 350 that had the high output ignition. I sure like how it started instantly in the bitter cold, seem that hot spark made a huge difference. Not so sure I liked this particular model car, but the build was both work and fun. Might have been an Accel igition kit for GM HEI if I recall. Not sure if there is anything like it for these tractors, but I sure as heck wanted to put one on that old truck and have it start so easy like that '78 did.