Need some servo help with my savage

My old steering servo went out so I got a new one (same as the original one) and it came today, so I was trying to install it and set everything back up. But the problem is that the notched plastic piece that comes out of the servo that hooks up to whatever you want to turn. (hopefully that makes some sense) Well that thing is not the same size as the servo that was in their originally and so the hole is too big for the thing that sits on the servo because the piece on the new servo is much smaller than the one that was on the servo that was in the truck.

How can I fix this? I haven't come across a problem like this before when replacing a stock part with the same stock part.

Thanks guys