Mine leaked twice thru the shift rod seal. The obvious other place is the driveshaft seal in the waterpump base.
If you have one of those gearlube pumps, you can use the fitting and plastic tube to test the gearcase for leaks. Drop the gearcase, drain the gear lube, screw in the tube and use a bicycle pump to pressurize the gearcase. Use some soapy water to find the leak, if it is not obvious.
The shift rod seal screws into the gearcase. A punch can unscrew it. I made a spanner out of a 3/4" black pipe coupler to tighten the replacement seal.
If the driveshaft seal is leaking, buy a new waterpump base and seal, and install it with the correct oring. Clean the driveshaft with sandpaper before installing the new water pump base.