I don't know what the piece of stock in the middle of the "bean" is but it gives you an option. Find a piece of black iron pipe that will slide over that round stock. It has a lot of wear on the top of it. Cut a slice of iron pipe, slide it over the round bar stock, and tack it on the end with the welder. That will help it take up some of the slop that has materialized in that cutout bean.
The other option is cut a new bean out of flat stock and weld it onto that one so the old one and new one are sandwiched together.
If nothing there moves the other option is push the seat forward and weld a piece of stock in there to take up the slack. I'm thinking of an old square nut or something. I'm guessing it moves frequently though, or it wouldn't be worn.