There's a little paper filter about the width of the back of a blister pack that goes in there to keep it from leaking. I'm pretty sure if you give it a quarter twist (and all the other ones that may be loose) it'll seal back up.
If you want to do a full repair look in my thread about rebuilding the transmission. I go through everything step by step. You can skip to the tailshaft portion but it will involve the bearings, which can be fun to get back in there. And you'll want a ring spreader thing like this
http://www.ebay.com/itm/LOCKING-RING...gAAMXQQQZRzoYL
I used two screwdrivers (flathead) the first time I did it and it was TOUGH. With the spreader you're done immediately.