You fill it by pumping oil in through the drain hole until it flows from the level hole.
It takes as much as it takes but it looks a very similar gearbox to the one on my slightly older 6hp. That holds 325ml. I expect yours will be either the same or a little less. (quoted in ml becase they are international and the same the world over. US ounces may be different to UK ounces)
Dont forget to use new washers on the drain and level plugs.
