Consider getting low cost UV light leak detection system. You add a UV dye to the crankcase oil.
Clean the area around starter very clean using something like Brake Clean.
Run the engine a few hours and then park in a dark spot.
Use the kits UV flashlight to see if any traces of the dye color you put into the crankcase appear where your leak is,
No sign of your dye then add a different color dye to transmission. Repeat.
Is tractor sitting outside and water is getting into bell housing around the clutch. A little oil will float on top of the water.
Check the drain hole or holes in the bottom of the clutch area.
Local autoparts places are good starting points for a UV kit with dyes for cool and, dyes for oil etc. They began life for AC systems but have broadened their uses since.
Dave