Hmmm, well some have a bearing. Spec number? On the Deere parts look up they don't show that, some did. So if you were to tear it down and replace the shaft and bearing- would it still leak? It just may be that this is the easiest place for a leak should the engine be pressurizing itself due to wear, other leaking seals, breather etc. or possibly fuel in the oil? If a person really wanted to install a seal, it could be done. How much work did you wish to put in this?