Update on the John Deere 111 tractor:
Last night, I was able to remove the engine and found the leaky seal. It's a round piece that goes around the cam shaft as it exits the engine and attaches to the drive belt. This little round guy has rubber to help hold the oil inside the engine. Seals, even on cars, tend to lose their oil holding abilities over the years and this one was no different. I pried the old seal out, and am ready to install a new one.
I also turned the tractor over and power washed the bottom. I know, I know... some folks say "Never wash your tractor," but this one is around 25 years old. The bottom was just caked with a greasy, grassy muck. The only way to get it off was to blast it. It now looks good as new, or at least clean.
I visited the local John Deere dealer at lunch and purchased a new seal, so I'm now ready to start reassembly.
I just hope I remember where all the pieces go and I that I don't have a bunch of important looking parts left over when I've got it all back together.