The "oil seal" is just the gasket between the pump and the pump adapter plate it bolts too. The water pump has an internal seal on its shaft which is the "water seal" which can be replaced if you can get the parts, and get the old seal out and press the new one in. make sure not to ding the shaft getting the old seal off. If your pump is old, the impeller plate is likely scored/worn and you might consider replacing the entire pump.
With the engine off, the oil level is someplace in the pan, so you wont get any oil OUT the hole when you take the pump off. What you *dont* want to do is get anything IN the hole. Especially water, as the hole goes inside of the timing gear cover (that whole front part of the engine) and connects to the oil pan. there is a flinger (thats what the parts diagram says!) on the crankshaft that spins and throws oil all over the gears in there, which is why you need an "oil seal". Its not under pressure, so just RTV would do. If your pump leaked water into the engine, drain the oil, add new, run a bit, repeat, until oil is clean, then get 'er nice and heated up.
Careful also, that there is a little adapter piece between the shaft in the engine and the shaft on the water pump. that'll fall out when you take the pump off, so watch for it. That spot on the engine is where the Kubota tractor had the hour meter mounted.. You can also get a different Oberdorfer pump model that bolts directly to the timing gear cover w/o needing the adapter plate or shaft-extension-adapter. The adapter (square hunk of metal the pump bolts to) sits on four studs on the timing gear cover, the nuts should be easy to back out since they've mostly been sitting in oil most of their life, do dont worry if you get the "other" oberdorfer.
