The leak wouldn't concern me, as much as moisture inside. If there was a lot of water in there and it's set a long time, the water and oil separated and that's not too good for bearings . Is the leak at a seal? I'd change the lube and run it let here leak , just keep it topped off . Once you are sure everything is good on the inside , repair the leak.

PRO TIP: I add 1/2 a bottle of STP oil treatment to manual gear boxes , manual transmissions, transfer cases and non-limited slip type differentials. I learned it from an old mechanic when I was a teenager . He said that's all the stuff is fit for , said it's not fit for an engine. Bet it would work great in a bush hog gear box.