OK to put you out of your lust for info. I need four of these, 2 forward, 2 aft. But having spent an hours searching for gears, they came to over a £100 I am now on a different track!
Basically I was going to fabricate the housing, have a through prop shaft with a bevel gear in the centre, then a down shaft with a matching gear to mesh together. The top end of the shaft would have matching gears to go to the motor drive. The whole unit would be revolved around at the top by a crown gear and suitable waterproof seals.