Walk Behind Lawn Mowers

I have an HU700F mower, and all of the nuts and bolts that attach the wheel to the frame (body) of the mower have fallen out after three years of use. I can replace the parts, but cannot get access to put the nut on the back of the bolt due to the black plastic insert that separates the blade area from the orange body of the mower. I cannot find any way to remove this black plastic insert, and there are no screws holding it in place.

I have two questions:

1. How do I remove and replace this black plastic insert so that I can get access to the other side of the hole?

2. What length and diameter of nuts and bolts are needed for this attachment?

I am attaching a picture of the issue below.

Matt Weyer
Powell, TN
mattweyer@gmail.com