The price  on the 1950 sounds a little high for the amount of TLC it needs, but I am constantly amazed by the way these 4 digit Olivers are going up in value. Those GM diesels are pretty reasonable to rebuild, if you do it yourself.

On the 1955, is the smell only present when using the PTO? That would narrow it down. The PTO clutch actually sits about as far forward as the rear axles inside the tractor, so it wouldn't transfer much heat to the outer case.  

I would think that since it takes more power to run the mower than it does to pull it, that it would most likely be the PTO clutch. There should be a breather on the top of the PTO unit. Shut the tractor down and get near it with your nose and see if the smell is stronger there. Sniff up by the main clutch and see what your nose tells you there. The PTO clutch is a wet clutch, so if it slipping, then the clutch smell should have an oily smell to it as well. The transmission clutch is dry and should smell like, well burnt clutch.