Arthur, I had the same leak and if it is coming from where there shifter shaft enters the transmission it is an easy fix as well.
I drained the fluid from the bottom drain plug then undid the one bolt that holds the shift lever in place.
It comes off easily and there is absmall lip seal that can be popped out.
The measurement are on the seal and I ordered a couple of replacement items from a Honda motorcycle shop :)
That replacement stopped the leak and has held for four years now.
The info I have is that the seal is 20mm ID, 26mm OD by 4mm thickness. The Honda part number is 91262-GBF-831
I keep those white oul-absorbent cloth "diapers" spread out under the engine and transmission (and in the genset pan) so I can quickly see if any oil is getting loose.
SM 209, Adagio