I use a regular small wrench. You might be able to take the striker and spring out and go at the nipple from the end of the frame with a nut driver. When you get the nipple out, put anti-seize on it and only put it in hand tight.
Nipples, they come in 2 different sizes. One is a standard 1/4 X 28 and other is a large diameter size. Not sure what the thread size is it maybe be 28.
I have 2 Yazels, each one is as above.
If yours is the large diameter nipple not sure where to find one, I have a friend who is going to get me a couple in June at Friendship.