I have the same engine
and the manual. I have not replaced this seal however.
Looking at the cutaway diagram of the engine
one thing is clear. The seal is actually recessed into the outer surface of the gear
case. Under the cup shaped cover as I'm sure you know. Two screws securing the cover.
The end of the shaft protrudes into this cover. However, there is something not clear to me. On the end of the shaft is "something" else. Hard to identify in the cutaway. But larger in diameter than the shaft and seal.
Looking at the disassembly section doesn't help. The cupped cover isn't mentioned and may come off with gear case cover. The drawing is at the wrong angle to tell. But the cam shaft end is visible and free of any other part in a drawing suggesting the case cover had just been pulled free.
It appears to me that the cupped cover can be removed without harm. Then this unidentified part would be visible. With the seal directly behind and removable depending on this "wtf is it part".
I'll take a pic of the cutaway if you like and the disasembly pic.
But it's not going to hurt anything to pull that cupped cover and look.