On my boat the battery negative and the wire to the negative buss bar at the panel are on the same bolt at the rear of the engine. I felt the ground wire to the panel was too small, a #12 I think. I paralleled it with #10 wire rather than try to replace it.
P.S. Found an old picture. I was wrong, the original was a #10. It is the white wire on the back of the head of my old engine. Now there is a new engine and two ground wires going to the panel.