Heck, they are so common that other than MAYBE the engine block and transmission case, you can build an entire tractor out of restoration parts. When I was growing up in the 70's there was something like a half dozen N Series tractors in a one mile radius of my house. I literally learned to drive on them. Last year I did a substantial amount of repair work on the one my uncle owns, and I also did a HUGE amount of work on the 860 Ford that My best friend's wife owns. Ironically, BOTH had been bastardized by some local jackass that fancied that he knew how to convert them to 12 volts. I ended up having to basically re wire both front scratch and build workable alternator brackets. I also learned how to fix the Marvel Schebler updraft carburetors so they don't leave puddles of gas on the floor. For the obligatory racing content, Marvel Schebler was originally Wheeler Schebler, and Frank Wheeler was one of the founders of the Indianapolis Motor Speedway.
We also had a gas Ford 841 Power Master with a circle style orange tree hoe mounted on it. When you were working it and had to refuel, when you shut down you would here the fuel tank cap start whistling from the remaining gas boiling in the tank. Lucky we didn't blow ourselves up. That was one tough tractor.