Some tractors have the problem of getting fuel into the motor oil

If diesel fuel is getting into the crankcase this is one place it can leak from

If this compartment is full of diesel when you remove the side cover here are steps to take it apart
First remove the two screws holding this U bracket on

Remove the spring clip holding the governor control rod

The control shaft can be pulled out now, you can see the control block slot inside the hole

Here we see the O-ring that could cause this leak

The swivel is removed in this picture to show that it is loose in the hole and able to turn

The swivel should be in this position when ready to re-assemble so it will line up with the slot in the control block.