I never worked on a Fairy OD, but looking were that pin is, you could try to remove the shaft that holds the shift fork, so the fork rotates out of the way or comes out. Then maybe you can get to the pin in question. My guess is you'll have remove the unit from the tcase to repair it. The...
I don't know anything about the Fairy overdrive, but looking at the pics and glancing at the manual it appears the O/D unit has a reservoir to hold gear lube,
so the idler shaft bearings are lubricated.